Gamasutra - Free-to-Play - What Are the Financial Rewards?

Gamasutra looks into the revenue that free-to-play micro-transaction MMOs generate, getting some specific numbers from Puzzle Pirates.  It certainly isn't a complete picture but it gives some insight:

As James blogged recently: "People often ask me, with a wary look such as you'd give a lunatic, 'Why do you dish out your numbers like this?' It's a good question. There are possible downsides, but they are limited; if a competitor looks at my numbers and then goes on to execute better than us, I don't think that has much to do with our numbers. They executed better, that's the hard bit. Well done to them.

"The upside," he continued, "is that the more information that circulates the startup and games community, the more people will share their data. This rising tide will raise all boats. If I can shame my fellows into parting with their data, we'll all benefit."

Indeed, James reveals that Three Rings' MMO Puzzle Pirates takes in approximately $50 each month from each paying user (ARPPU) for a total of $230,000 a month, all resulting from microtransactions.