Question: is a fixed cost always a sunk cost

Answer: No, Sunk costs are sometimes contrasted with prospective costs, which are future costs that may be incurred or changed if an action is taken. In that regard, both retrospective and prospective costs could be either fixed costs (continuous for as long as the business is in operation and unaffected by output volume) or variable costs (dependent on volume). However, many economists consider it a mistake to classify sunk costs as ``fixed'' or ``variable.'' For example, if a firm sinks $400 million on an enterprise software installation, that cost is ``sunk'' because it was a one-time expense and cannot be recovered once spent. A ``fixed'' cost would be monthly payments made as part of a service contract or licensing deal with the company that set up the software. The upfront irretrievable payment for the installation should not be deemed a ``fixed'' cost, with its cost spread out over time. Sunk costs should be kept separate. The ``variable costs'' for this project might include data centre power usage, for example.