These are good questions to ask, but you're getting the XNA framework confused with XBox Live Arcade. They are not the same things. While you can make a game that plays on the 360, it is not possible at this time to sell the game or make it downloadable for everyone. So there is no current revenue stream on the 360.
You can sell games you make using the XNA framework on the PC, but there is not standard distribution system for this and the numbers you are asking for vary dramatically for games and portals that sell games.
What you should do is use the XNA framework to build you game so you can demonstrate your game idea is a good one and is fun to play. Then a publisher can pick up your game and at that point you can distribute it through XBLA. But that's going to be a ways down the road if you haven't yet made a game.
There has been talk of a community games system that was mentioned at the Games Developer Conference, but there hasn't been any more information from Microsoft yet about how this system is going to work.
Hope that clears some things up for you.
George Clingerman - Microsoft XNA MVP
XNADevelopment.com
