Why the Cost Category Matters
A business deciding whether to produce an additional batch, continue an order, or reduce output should not treat every amount in its accounts as a reason to keep producing. Some costs have already been incurred and cannot be recovered. Other costs will be incurred only if production continues. Still others will be paid either way. The decision depends on which items actually change between the alternatives.
This distinction is useful whenever a choice is forward-looking: the relevant question is not simply, “How much has the business spent?” It is, “What will be gained or avoided by choosing this output rather than the alternative?” A cost’s classification depends on the decision and its time horizon. The same expense may be avoidable in a long-term decision but unavoidable under a short-term contract.
These terms describe different features. “Sunk” concerns whether the cost can still be changed or recovered. “Avoidable” concerns whether a future cost can be prevented by a particular decision. “Relevant” concerns whether the amount differs across the alternatives. A cost may be fixed for a production period yet avoidable if the business can cancel it by closing; a variable cost may be unavoidable if a binding contract requires payment regardless of output.
Compare Alternatives, Not Accounting Totals
Suppose a firm is choosing between alternatives A and B. Let \(R_A\) and \(R_B\) be the revenues from those alternatives, and let \(C_A\) and \(C_B\) be their economic costs. If a cost has already been incurred, or is otherwise identical under both alternatives, it appears on both sides of a comparison and cancels. The difference in net benefit is therefore determined by the differences in revenue and relevant cost.
This comparison can include an opportunity cost: the value of the best alternative use of a resource that the decision gives up. For example, if producing one order uses capacity that could earn a contribution from another order, that forgone contribution is a cost of choosing the first order. It is not necessarily a cash payment, but it changes the economic comparison.
For example, compare producing 500 units with producing 400 units during the next month.
Include future revenues and costs that differ between the alternatives, and any opportunity cost of resources used.
Exclude sunk costs and future costs that must be paid under either choice.
State which alternative is better on the specified financial comparison and by how much.
Worked Applications
Worked Example: An Additional Production Batch
A workshop must choose between producing 400 or 500 customized storage units this month. Each unit in this order will sell for $24. The 100 additional units require $11 per unit in materials and labor. Producing 500 units also requires a $700 setup service; producing 400 does not. A $4,000 design study was paid for last month and cannot be refunded. Which output has the higher net benefit for this order?
Method. Compare 500 units with 400 units. The design study is sunk: it has already been paid and is unchanged by the current choice. It should not be assigned only to the additional batch.
Additional revenue. The extra 100 units bring in \(100\times \$24=\$2,400\).
Additional avoidable costs. Materials and labor for the extra units cost \(100\times \$11=\$1,100\). The setup service adds $700. Total additional cost is \(\$1,100+\$700=\$1,800\).
Difference in net benefit. Producing 500 rather than 400 units adds \(\$2,400-\$1,800=\$600\). On these figures, the 500-unit choice has the higher net benefit by $600.
Check and common mistake. The incremental calculation includes the $700 setup charge because it occurs only with the larger output. It excludes the $4,000 study because that payment cannot be changed by this decision. Charging the study only to the 500-unit option would give the incorrect calculation \(\$600-\$4,000=-\$3,400\). The study was paid under either output choice, so it cannot make the smaller batch relatively better.
Worked Example: A Cost That Is Unavoidable This Month
A packaging firm is considering cutting monthly output from 1,000 boxes to 800. Each box sells for $20. For each box produced, materials cost $7, a packaging fee costs $2, and a sales commission costs $1. A staffing contract requires the firm to pay $6,000 this month regardless of output, and the remaining lease payment for the month is also due whichever output is chosen. An earlier portion of the lease has already been paid. What is the net effect of cutting output by 200 boxes?
Method. Compare the 800-box choice with the 1,000-box choice for this month. The staffing contract and remaining lease payment are unavoidable within the stated horizon: neither changes when output falls. The earlier lease payment is sunk. None of these amounts belongs in the incremental comparison.
Revenue forgone. Cutting 200 boxes means giving up \(200\times \$20=\$4,000\) of sales revenue.
Costs avoided. Materials, packaging fees, and commissions total \(\$7+\$2+\$1=\$10\) per box. The reduction avoids \(200\times \$10=\$2,000\) of these costs.
Net effect. The cut gives up $4,000 in revenue and avoids $2,000 in costs, so its net effect is \(-\$4,000+\$2,000=-\$2,000\). The firm is $2,000 worse off financially by cutting output, based on the costs and revenues given.
Check and common mistake. The $6,000 staffing payment does not fall when output falls, so treating it as a $6-per-box saving would be incorrect. A payment can be fixed and still avoidable in another decision or time horizon; here, the contract makes it unavoidable for the month being considered.
Worked Example: Cancellation Penalty and a Sunk Setup Fee
A small manufacturer has the option to complete a special order for 600 components or cancel it. Completion produces $27,000 in revenue and requires $5,100 in materials, $2,100 in labor, and a $900 equipment rental charge. Canceling incurs a contractual penalty of $1,200. The manufacturer has already paid a nonrefundable $2,000 setup fee for the order. Compare completing the order with canceling it.
Method. Compare the two current choices. The setup fee has already been paid and cannot be recovered, so it is sunk under both alternatives. The cancellation penalty is a cost of canceling; the materials, labor, rental, and sales revenue apply if the order is completed.
Net benefit of completing. Revenue is $27,000. Costs incurred by completing total \(\$5,100+\$2,100+\$900=\$8,100\). Thus completion yields \(\$27,000-\$8,100=\$18,900\) before the common sunk setup fee.
Net benefit of canceling. Canceling avoids the completion costs and revenue but incurs the $1,200 penalty, giving a net benefit of \(-\$1,200\), again excluding the sunk setup fee.
Comparison. Completing rather than canceling improves the comparison by \(\$18,900-(-\$1,200)=\$20,100\). Thus completion is better by $20,100 on the stated figures.
Check and common mistake. The penalty is not a cost of completing; it is avoided by completing. Equivalently, the completion advantage is \(\$27,000-\$5,100-\$2,100-\$900+\$1,200=\$20,100\). Subtracting the $2,000 setup fee from completion alone would incorrectly treat a past, unrecoverable payment as if canceling could recover it.
Why the Comparison Works
The rule follows directly from subtracting the net benefit of one option from that of another. Write each option’s net benefit as its revenue minus its costs. If a cost \(K\) is incurred under both A and B, its contribution to the difference is \(-K-(-K)=0\). The same cancellation applies to revenue that is identical across both options. Only differences remain.
If the costs include a common sunk amount \(K\), write \(C_A=K+C_A^*\) and \(C_B=K+C_B^*\), where \(C_A^*\) and \(C_B^*\) contain the costs that may differ. Substitution gives:
The \(K\) terms cancel. This is why a sunk cost cannot change the ranking of the alternatives. The argument does not say that the business never has to report the cost or that it was unimportant when first deciding whether to spend it. It says only that the unrecoverable amount cannot be changed by the current choice.
Common Traps and Practical Checks
A reliable analysis asks what changes, not whether a cost appears under a heading such as “fixed,” “variable,” or “overhead.” A fixed cost may be avoidable if a lease can be canceled, a shift can be removed, or a facility can be closed. Conversely, a variable-looking charge may remain payable under a minimum-purchase agreement. Read the conditions of the decision carefully.
- Including past spending to justify continuing. Money already spent cannot be recovered merely by producing more. Do not let a sunk expense make an otherwise unfavorable future choice look attractive.
- Assuming every fixed cost is irrelevant. A fixed charge that disappears under one alternative is relevant, even if it does not vary with each unit of output.
- Assuming every variable cost can be avoided. A binding commitment may require payment even if production stops. Check the contract and the time period.
- Ignoring the cost of the next-best use. If production ties up capacity that could be used for another profitable activity, the forgone alternative may be an opportunity cost.
- Comparing totals from different baselines. State both alternatives and use the same time horizon. Include a cost under the option that causes it; do not subtract a cancellation charge from the wrong option.
Check Your Understanding
For each problem, identify the relevant changes before comparing the alternatives.
- A workshop compares producing 200 or 250 lamps. Each lamp sells for $18. The additional 50 lamps require $6 each in materials and a $2 packing charge per lamp. A $500 inspection fee applies only if 250 lamps are produced. Find the change in net benefit from choosing 250 rather than 200 lamps. (Answer: $0; the choices are financially equivalent on these figures.)
- A supplier considers reducing output by 100 units. Each unit sells for $31 and uses $9 of materials plus a $1.50 output-based fee. Salaries of $8,000 are payable either way. Find the net effect of reducing output, excluding any other changes. (Answer: Net benefit falls by $2,050.)
- A business can complete a project for $27,000 revenue. Completion requires $11,400 in materials and labor and a $1,600 rental charge. Cancellation requires a $2,500 penalty. A $3,000 feasibility study has already been paid and cannot be recovered. By how much is completion better or worse than cancellation? (Answer: Completion is better by $16,500.)
- A plant can operate for a month or close. Operating earns $16,000 in revenue and requires $6,500 in materials and $3,000 in labor. A $4,200 rent charge is canceled if the plant closes. Closing also requires $1,100 in severance payments. A $2,000 lease payment has already been made and is nonrefundable. Compare operating with closing. (Answer: Operating is better by $3,400.)
- A producer can accept a special order that earns $12,500 and requires $6,800 in avoidable costs. Accepting it uses capacity that could instead earn $2,400 in net benefit on another order. A $900 setup fee has already been paid and cannot be recovered. Compare accepting the special order with using the capacity for the alternative order. (Answer: Accepting the special order is better by $3,300.)