According to Edge-Online, World of Warcraft made up half of Activision Blizzard's earnings last year:
Arvind Bhatia with Stern Agee said in a Wednesday research note that he expects WoW subscriptions amounted to earnings per share of about 30 cents out of a total 60 cents, or around $400 million total, in the fiscal year ended in December.
That means that Wow makes as much money as Call of Duty, Guitar Hero, Starcraft, Crash Bandicoot, Spyro the Dragon, and the rest of Activision's franchises and games combined. So if you're ever wondering why companies keep creating new MMOs to challenge Warcraft, there's your answer. There's a lot of money in it.