Grrr.... Typed out a big reply to this and it didn't post...
Anyway, the gist of it was that this is actually a pretty solid idea. It should make people buy more coins with real money, and it should cut down on people hopping out of game modes they don't like. It would, however, make it irritating to switch to the losing team to give them a chance, and to try to find a game w/ a particular person.
There's two other issues, but these can be mathematically solved.
#1: This would give an advantage to people who already have purchased upgrades& planes, and earned coins, as it will now be harder to earn x amount of coins. This could be solved by either taking stuff away from people who are already around (Don't do that, please.

), OR, you could shift the prices of everything down, and cut how much $$$ people have in the bank based on some sort of conversion factor.
#2: You couldn't charge a flat rate of $500, or any flat rate. This would utterly devastate noobs, and have no effect at all on us higher-ups who don't need money, or have no trouble getting $500 several times over in a game. You would have to adjust the join-fee based on a ratio that could be derived from your level, k/d, upgrades and planes already purchased, amount of money in your account, and maybe even ratio of shots into an enemy to shots into a blue.
Actually, Might, this was a pretty good idea!