Paying for MMORPGs is perfectly reasonable in my opinion. Servers, while getting cheaper as technology improves, still aren't that cheap. Also, Blizzard does need to get paid for improving the game content so much. Although I would agree that $15 a month is a bit steep. Halving that rate or even reduction to $10 a month would seem more reasonable in terms of comparable RPGs, although
1) Blizzard won't risk losing profits to gain more players, they already are the biggest MMORPG out there.
2) I don't know their finances, and they need the money for Starcraft II and Diablo 3.