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Identifying the proper unit costs of aircraft is a real challenge for the expert, much less the average Wikipedia editor. This is quite understandable since there are so many different ways of reporting aircraft costs, as well as the fact that most sources reporting costs mislabel the specific type being reported (usually through ignorance as well). Moreover, two different countries might have similar, but differently named and slightly differently calculated costs that are otherwise roughly equivalent. Actually discerning whether a given cost number is accurately referenced requires some experience, though, especially since sources generally aren't very clear as to what cost elements are or are not included. To aid our editors in understanding what they're reading (assuming the source is knowledgeable), I'd like to provide a brief overview of the different ways in which costs are typically captured and reported (or mis-reported).
There are two key elements to understand about reported costs you may encounter. One is the "type" of cost calculation and the other is the "year-basis" of the given value. The unit cost should be expressed in the format of "US$75 million (2002)". The reason is that your source may be supplying a value that has since changed due to the effects of inflation or fluctuating currency exchange rates. Furthermore, it would be a prodigious task to update the quoted prices for every aircraft Wikipedia has an article about.