According to this ars technica article, average earnings on Xbox Live Indie Games is less than $4000. Total.
Now, average can mean a lot of things... if a few people are doing hugely well and most people do terribly, the average number is largely meaningless. Also, as usual, how good or bad that money is depends on what time and money you put into writing the game in the first place. Some people are absolutely thrilled to earn a few hundred dollars.
Now that I have to pay rent and feed people, though, a few hundred wouldn't really do me much good. Luckily my projects do generally pull in rather more than $4000 each. :)