Up front PC costs can be fairly high. But, unlike consoles where you junk the old one after five years, you can typically upgrade PCs incrementally. Need a little more graphics power? Buy a new video card for $250. Swapping to hard disk too often? More RAM, $100. Out of space? Hard drive, $50. The *big* shifts in computer hardware happen (relatively) infrequently; most of the time its similar hardware that performs better. Can't say the same thing for consoles, where your $600 PS3 is useless once the PS4 comes out. Most of the non-essential PC components also have the advantage of being once and done; I don't *need* to buy a new keyboard, mouse, power supply, case, or sound card for a long time after I get my first one. Again, can't say the same for consoles, as I need to buy stuff like controllers and memory cards with each new console iteration.
Now, not everyone is willing to take the time/is smart enough to build their own PC out of parts, incrementally, but it's a huge cost saving, and reuseability is a distinct advantage over consoles. Not to mention that a good PC can be used for an awful lot of different things besides gaming.
Addendum: The 2.4 GHz Pentium 4 (ie, what you need to run Bioshock) first appeared in 2001, the same year as the XBox. Something tells me you couldn't run Bioshock on the XBox...which makes it sound like you need to upgrade *consoles* more often than *PCs*, not the other way round.
