Lightspeed, a venture capital firm, checked out the average revenue per user for some casual, free to play MMOGs (Club Penguin, Habbo Hotel, Runescape, and Second Life), establishing that other that SL - which pulls in a little over $9 a month in ARPU, thanks mostly to virtual land upkeep - these sorts of casual MMOGs pull in around $1.25 ARPU per month. Which, until one considers the user bases of games like Club Penguin, seems like a damn hard way to make some money:
Second Life: $9.30/mthly user/mth
Club Penguin: $1.62/mthly user/mth
Habbo: $1.30/mthly user/mth
Runescape: $0.84/mthly user/mth
Having spoken to many other MMOGs and virtual worlds on a private basis, this estimate seems to be a good gauge for what a well performing MMOG can aspire to from a free to play business model.
Left out are a number of other popular MMOGs like MapleStory; I'd be curious to see firmer statistics for a wider spread of games, though the comments section does include some speculative numbers for a few other games.