I agree with most guys here:
getting the hardware is one thing. I already had 3 computers when I decided to 5 box. Two of them are pretty old but wow needs so little performance they work okay. I was comtemplating buying a laptop anyways so my only real investment was getting a second laptop.
For the money I need for the accounts I could take my girlfriend to dinner about one or two times and taking one hour of a guitar/golf/whatever lesson nets more than the money I invest in a wow account in a month.
So actually it is pretty cheap.
Bunny