I have a suggestion related to this issue (which is why I didn't post it in the suggestions board), don't know if something similar got suggested already:
Pair the future money use (of buildmode) with vehicles and race mode quotas. Vehicles and race modes will have quota brackets.
For example supercars each have 3% to 5%, IV races 4% to 6%, SK 10% to 20%, strange vehicles 0.05% to 0.1%... these are random examples of numbers, might not be the best.
So if the number of races with the vehicle/mode if over the high bracket, the cost of making a race will increase (exponentially in order to make it extremely expensive if it's way over the quota). And same if the number of races are under the quota, make it cheaper.
It might take a bit of time to set quotas for vehicles and race modes but in the end it may be fun (and useful).