Gamers.com has an article on some MMORGs that are in the works on various platforms accompanied with some stats on how much revenue some older titles are pulling in. Yahoo also have a similar article here
"Even with its limited audience, 'EverQuest' is generating $40 million-$50 million a year for a couple of years, which is a great return on an investment that's much less than developing a movie"
There is also another article which makes some analysis of MMOG Subscription Growth, and comes up with some startling claims:
1. DAoC (Dark Age of Camelot) did not substantially add to the MMOG customer base, but instead stole subscriptions from Everquest and Ulimate Online.
2. MMOG expansions do not serve to increase a given MMOG's customer base.