One of my housemates has a PS3, I have my PC, so I guess I'm in a good position to compare the two.
The PS3 is great if you have flatmates that you could potentially play multiplayer games with, and seriously, the PS3-has-no-games argument is no longer valid; there are plenty of decent games for it these days.
My housemate uses his PS3 for loads of stuff; playing music, watching movies and tv he's downloaded, it's a multimedia platform that you can really share, and while I can do everything just the same on my PC, it just somehow seems like a more personal experience than one for several people.
Alot of the time people don't take into account the fact that a console requires a TV when comparing prices, and this can sneak away a fair bit of cash from your wallet if you don't already have one; PC monitors tend to be cheaper.
As far as prices for building a PC goes, it depends on a few things; how powerful do you want it to be? (What games do you plan on playing on it?)
My own rig, for example, can manage Crysis, (and thus every other PC game), on maximum without much strain at 1440x900 res, and the whole setup, including monitor, peripherals etc, was just under £700, though you could probably knock £50 or so off the top of that since prices have dropped somewhat.