Your calculations suck and your methodology is wrong. First and foremost, 8x15 is 120mil, not 1.2bil. Secondly, the Chinese players pay significantly less than $15/month due to their per-hour pay system and lower cost of living. Monthly revenue is likely closer to 80mil/month at best, and it's anyone's guess what operating expenses are.
Even still - that would be 1.4 billion, per year, without adjusting the conversion of foreign currencies. If you factor in foreign currency adjustment and guess (sorry - cant do much more than that from where I stand
), this would drop the total to a quarter of that number. 350 million per annum.
This is an insane amount of money for any developer. Even a quarter of this would be a gold mine in any developers financial interest (87.5 million). Including running costs, this must be making a massive profit for Blizzard.