Jeff Tunnell, co-founder of GarageGames, has posted a very in depth and interesting look at the numbers behind XBLA. While I won't go too in depth in it here the basics (as far as he could tell them without breaking NDA) were this:

• Marble Blast Ultra cost close to $300,000 to develop
• It sold approximately 120,000 copies
• At $10 USD a piece, it made approximately 1.2 million
• Developers get to keep between 30%-65%
• At 65% (his number not mine) they brought in approx. $780,000

He also mentioned that while MBU is considered one of the highest grossing XBLA titles, it would be considered a failure to some publicly traded studios who look for at least a 5x return on investment.

 

[UPDATE] It seems that I forgot to add the link. You can check it out here.