Agree with what's been said. Despite paying a small fortune to Blizzard for 'one game' (plus pets, mounts etc) I have saved money elsewhere.
That is not to say I simply sit at my computer 24/7 but I am not out drinking etc every night for entertainment. Also, it keeps me out of trouble!
As long as you make good use of it I think that its good value for money considering the time spent on it!