Well... since people are mainly looking for multiplayer games and xn/a doesn't allow it on xbox360, you probably need to go the xbox live arcade way (not using xn/a) to try to have MS pay you for your game (but it's still something reserved to professionals).
Anyway, here are some fresh news :
Prize list finally appeared...
Apparently MS failed in convincing some sponsor to offer juicy prizes. The official prizes (on Wed. Feb. 14th) are five $500 prizes. Mmm... I guess you need to substract the club subscription fee from that...
You also need to be a professional lawyer to analyze the rules of the contest (long long text). That text says that, eventually, some sponsor will replace these prizes with other prizes, later...
For sure, MS is allowed by you to make some profit with some advertising based on your game snapshots.
Well, I guess we can't ask more from a game division still in the red...