if your plan is not to make money, but to make Tremulous more well-known (which is doubtful), then make it clear, to all buyers, that "this game is free and open-source, and can be downloaded from <...>" (also, optionally, that "if you have payed for this game, then you have been ripped off"), and lower the cost of the product as soon as you have covered all Appstore expenses.
also, i'm not attacking your decision to cover costs, but your illegal actions of violating the GPL (the copy of the license should have been included, the source code should have been made directly available, etc.), which i have not verified, but have heard rumors about.