I sat in on the SXSW Session on selling virtual goods and wanted to point to a couple of links that might help the aspiring seller to understand the economics of doing this:

My Best Estimate for the Size of the Virtual Goods Market in the United States by Charles Hudson

http://www.charleshudson.net/my-best-estimate-for-the-size-of-the-virtual-goods-market-in-the-united-states

ARPU vs ARPPU  by Raph Koster

http://www.raphkoster.com/2009/03/16/arpu-vs-arppu/