First, it is not about cost, it is about not liking the game. Basically, since you don't enjoy game now, you won't enjoy it anyway. And a possibility to play it for free means you'll be only a leech player, never contributing anything to Blizz, just reserving a spot on the server for free.
Second. You for sure won't stop eating bread, or buy fuel for your car, or spend money in cinema etc etc just because in a year the overall cost will be equal to a couple games on Steam, will you? Also, 180$ in a year is as much small sum as 15$ in a month. Really, I'm wasting more money for a printer paper per year, lol.
It is so funny to watch people spent tons of cash buying a new cell phone every month or a new car every year and yet bitching about 15$. (And I'm not talking about you, Jtbrig7390, in this last statement - just for clarification).