I have heard it said that a good shooter can out shoot others that have:
1.better equipment
2.better ammo
3.less experience.
If all that is true or even partially true you can solve the ammo cost problem by providing the ammo for each match. A good shooter should beat a not so good shooter with Blazer @ 79 cents/box. Every match will feature some cheap ammo. I know every guns prefers a certain ammo but a good gun will usually group most ammo well.
Truth is there are a lot of perfect scores showing now. If ammo was $1000/box and I could shoot a perfect score I might buy some just to do it but when everyone can do it the thrill is gone.
If you want to really make a fuss at some match, require all the contestants to put their ammo in a drawing and each competitor will draw for it. You won't know what ammo you are shooting until you get it! The expensive ammo will not show up there would it? If the rifles and flags get too expensive, you can draw for those too. Is this what you want? Just thinking out loud.