A relevant cost is a cost that differs between alternatives being considered. It is often important for businesses to distinguish between relevant and irrelevant costs when analyzing alternatives because erroneously considering irrelevant costs can lead to unsound business decisions. Also, ignoring irrelevant data in analysis can save time and effort. Non-cash items, such as depreciation and amortization, are frequently categorized as irrelevant costs, since they do not impact cash flows.

Two common types of irrelevant costs are sunk costs and future costs that do not differ between alternatives. Sunk costs are unavoidable because they have already been incurred. Future costs that do not change between alternatives are also essentially unavoidable with respect to the alternatives being considered.

Example

A construction firm is in the middle of constructing an office building, having spent $1 million on it so far. It requires an additional $0.5 million to complete construction. Because of a downturn in the real estate market, the finished building will not fetch its original intended price, and is expected to sell for only $1.2 million. If, in deciding whether or not to continue construction, the $1 million sunk cost were incorrectly included in the analysis, the firm may conclude that it should abandon the project because it would be spending $1.5 million for a return of $1.2 million. However, the $1 million is an irrelevant cost, and should be excluded. Continuing the construction actually involves spending $0.5 million for a return of $1.2 million, which makes it the correct course of action.