it really depends
how much RAM do you want, how much Disc Space, are you going for a Hard Disk Drive or Solid state Drive (or in some cases both), what graphics or wireless card you want, how many expansion ports or USB 2.0/3.0 ports...
I would recommend a HD Display Monitor, or hook it up to an HD TV if your TV has the required ports..
PCs are not as simple to purchase as Consoles, the price is going to entirely depend on Specs, Brand, and a few other factors...
if you are not really aware of what your going to need hardware wise, you can always call up your local PC Shop (such as a Futureshop or Staples retailer) and inquire, they will be more and glad to give you some advice on what will be able to play the type of games you want and can easily tell you how much it will cost (and if you don't mind paying a little extra they will likely Build a PC for you which would be more powerful then most things they have in stock (I used to work at Futureshop building Custom PCs for customers))
I agree, don't throw the money required for a decent gaming rig just for one game, only do it if you really wanna switch to PC gaming in general