Gamasutra takes a look at how much money micro-transactions can generate for developers of free MMOs, you can read it over here.
"We just jumped in," he recalls. "There were no data points and, frankly, every game is different, every play population is different, and extrapolating from one developer's data to your own is, well, an interesting intellectual exercise but it doesn't necessarily tell you what to expect."
Four and a half years later, James has learned a lot -- that the average revenue per user (ARPU) is between one and two dollars a month, but only about 10% of his player base has ever paid him anything. As a result, he says, approximately 5,000 gamers are generating the $230,000 in revenue he sees each month.