A cost that is relevant in one decision may not be relevant in another decision.

See Also:
Sunk costs: How should they affect your future business decisions?
Variable vs Fixed Cost
Replacement Costs
Joint Costs
Service Department Costs
Ten In-House Secrets for Reducing Your Company’s Legal Costs
Capital Budgeting Methods

Sunk Cost Definition

What is sunk cost? A sunk cost is a cost that has been incurred and cannot be recovered. The money is spent. In accounting, a sunk cost is a type of irrelevant cost. When facing a potential project or investment, a manager must only consider relevant costs and ignore all irrelevant costs.
When a manager is considering a particular decision, relevant costs are the costs that are incurred if the decision is made and irrelevant costs are the costs that are incurred whether or not the decision is made. A sunk cost is not a relevant cost for decision making.
Whether a cost is relevant or irrelevant depends on the decision at hand. A cost may be relevant to one decision and that same cost may be irrelevant to another decision. A sunk cost, however, is always an irrelevant cost.

Sunk Costs Fallacy

The sunk cost fallacy is when someone considers a sunk cost in a decision and subsequently makes a poor decision.
An example of the sunk cost fallacy is paying for a movie ticket, finding out the movie is terrible, and staying to watch anyway just to get your money’s worth. When you find out the movie is terrible, you should make a decision whether to sit through the bad movie or to do something more meaningful with your time – the price you paid for the ticket should not affect your decision. The ticket price is a sunk cost.
Another example of the sunk cost fallacy is paying for an all-you-can-eat buffet, eating until you’re full, and then going back for more just to get your money’s worth. When you are full, you should decide whether you want to eat more or to stop eating – the fact that you paid for unlimited food should not affect your decision. The price of the buffet is a sunk cost.

Sunk Costs Examples

Let’s say a company spent $5 million building an airplane. Before the plane is complete, the managers learn that it is obsolete and no airline will buy it. The market has evolved and now the airlines want a different type of plane.
The company can finish the obsolete plane for another $1 million, or it can start over and build the new type of plane for $3 million. What should the managers decide? Should they spend that last $1 million to finish up the plane that’s almost done, or should they spend the $3 million to build the new plane?
At first glance, you may think the company should just finish the old plane. It’s only another million bucks and they already spent $5 million. But in reality, the five million is irrelevant. It is a sunk cost. The only relevant cost is the $3 million dollars. The managers should consider whether or not to spend $3 million on the new plane, and nothing regarding the old plane should affect the decision.

What Is Relevant Cost?

Relevant cost is a managerial accounting term that describes avoidable costs that are incurred only when making specific business decisions. The concept of relevant cost is used to eliminate unnecessary data that could complicate the decision-making process. As an example, relevant cost is used to determine whether to sell or keep a business unit.

The opposite of a relevant cost is a sunk cost, which has already been incurred regardless of the outcome of the current decision.

Key Takeaways

• Relevant costs are only the costs that will be affected by the specific management decision being considered.
• The opposite of a relevant cost is a sunk cost.
• Management uses relevant costs in decision-making, such as whether to close a business unit, whether to make or buy parts or labor, and whether to accept a customer's last-minute or special orders.

Example of Relevant Cost

Assume, for example, a passenger rushes up to the ticket counter to purchase a ticket for a flight that is leaving in 25 minutes. The airline needs to consider the relevant costs to make a decision about the ticket price. Almost all of the costs related to adding the extra passenger have already been incurred, including the plane fuel, airport gate fee, and the salary and benefits for the entire plane’s crew. Because these costs have already been incurred, they are "sunk costs" or irrelevant costs.

The only additional cost is the labor to load the passenger’s luggage and any food that is served mid-flight, so the airline bases the last-minute ticket pricing decision on just a few small costs.

Types of Relevant Cost Decisions

Continue Operating vs. Closing Business Units

A big decision for a manager is whether to close a business unit or continue to operate it, and relevant costs are the basis for the decision. Assume, for example, a chain of retail sporting goods stores is considering closing a group of stores catering to the outdoor sports market. The relevant costs are the costs that can be eliminated due to the closure, as well as the revenue lost when the stores are closed. If the costs to be eliminated are greater than the revenue lost, the outdoor stores should be closed.

Make vs. Buy

Make vs. buy decisions are often an issue for a company that requires component parts to create a finished product. For example, a furniture manufacturer is considering an outside vendor to assemble and stain wood cabinets, which would then be finished in-house by adding handles and other details. The relevant costs in this decision are the variable costs incurred by the manufacturer to make the wood cabinets and the price paid to the outside vendor. If the vendor can provide the component part at a lower cost, the furniture manufacturer outsources the work.

Factoring in a Special Order

A special order occurs when a customer places an order near the end of the month, and prior sales have already covered the fixed cost of production for the month. If a client wants a price quote for a special order, management only considers the variable costs to produce the goods, specifically material and labor costs. Fixed costs, such as a factory lease or manager salaries, are irrelevant because the firm has already paid for those costs with prior sales.

What costs are relevant to decision making?

Which one of the following costs is not relevant for decisions?

What 2 types of costs are never relevant to a decision?

Which of the following statements is true fixed costs may be relevant in a decision avoidable costs are irrelevant costs in decisions?