Previous cost to increase pilot skill... great!
Going from 3 to 4 would cost three. Going from 4 to 5 would cost 4...
Now to make it a little easier past 5... going from 5 to 6 costs Four. Going from 6 to 7 costs three. 7 to 8 costs two. 8 to 9 costs one.
Depends on how easy it is to accrue 1 point to use to buy stuff. This model does a great job of prioritizing getting ace status but after you achieve that (PS5) the gradations are much smaller and when you're almost the best becoming "the best" is really more about the "best" dying and not really about you improving any farther. Could even have a rule that you can only have 1 PS9, 2 PS8, 3PS7. Could be fun.