You can set an amount that you can spend and you can't go over. This could add new challenges and "budget planes".

Some additions could be that the longer the fly for or the distance/height you go you get more money to spend on your next plane.

Also in multiplayer (if it comes out) the host/server could set an amount that the players can spend making the fight a bit more fair. Of course both players would have to have extensive knowledge of how to build planes but it could be a really cool idea.