First, are the programs insanely expensive is a question that needs to be asked. I have no doubt there is waste to cut, but overall, a modern fighter much be complex. A WWI fighter will lose against the more complex fighter every-time, which will lose to the 1950s fighter (The jet engine was just becoming workable at the end of WWII, if the war had gone longer what I'm calling a 1950s fighter would be late WWII). And so on. A modern fighter must have high R&D, and overall we hope to only need a relatively small number of them, which makes it really easy to do division on a per fighter basis and get a really big number.

The incremental cost of a F35 once you have designed it isn't too bad (and it could be made a lot better if it was worth a larger assembly line to make more).