Money is one factor, but so is time. A lot of these kinds of parts like heat shields and expensive instruments are effectively one-offs, where there may be 1, 2 or 3 of them made, and usually the test articles aren't fit for flight because they determine defects that are later fixed in the one that flies.

There are certain launch windows for Mars where Earth and Mars are closer together [1] that you really need to be ready for. If you miss it, you might have to wait 2 more years for the next window. Sometimes you can't get the necessary refit time no matter how much money you spend.

[1] https://en.wikipedia.org/wiki/Exploration_of_Mars#Launch_win...