Techniques of Systems Analysis notes that there are almost always large uncertainties when it comes to costs:
At first sight, costs look like a pretty concrete thing. You just grab an accountant and put him to work. However, as always, a close examination makes the precise imprecise. As we will explain later, when we talk about time-phasing, there is a real ambiguity in deciding the dollar cost of a system. Roughly speaking, this occurs because a military system is not bought at one instant of time by going into a department store and ordering it. It has to be built up over the past years and it is expect to have a continuous existence in the future. Under such circumstances one must always ask himself what it costs to use facilities which are already owned, and what will be the salvage value of any expenditures made this year in future years. Also, if one is procuring or developing a new system, he may have had no experience on which to base cost estimate.s It is surprising, in practice, how inaccurate even careful estimates of the costs of new systems have proved to be. Careless estimates tend to be out of this world.
There is another ambiguity in costs which the Analyst generally ignores but with which the policy make is sometimes concerned. Some dollars are harder to come by than others. Research and Development funds, for example, are ordinarily tighter than procurement funds. In the U.S., expensive gadgets are often easier to buy than high grade, but relatively low cost, people. Public works funds, for obvious reasons, are often easy to get and, of course, traditional expenditures are almost always easier to justify than new ones.
Finally, there are many costs which are not usually measured in dollars, such as crew lives, dislocations, political effects, etc.