The thing is, consoles have a lot of little fees tagged on everywhere that really add up. Incredibly high game prices, they have to pay for DLC, subscription fees in the case of XBox
I actually did some calculations of my own, and PC is cheaper than XBox 360 in the long run. Yeah, and you don't need to upgrade your PC all the time. Mine has been happily running even new games for 2 years now (or, since I built it).
Thing is, on PC, you may have a hefty down payment, but from then on, it's a very inexpensive platform. And that down payment may not even be that huge. Some people may just decide to pick up a 9800gt (more than enough to run most modern games) for their existing PC. Some may have needed a new PC anyway, and decided to put out the extra 200 for a video card (that was my situation). If you look at it from the standpoint that you're just paying for a new PC plus a bit extra for a video card and maybe a nice mouse.
From my standpoint, I only spent $150 on gaming. I needed a new PC anyway for regular PC things.
After that, you have free multiplayer, inexpensive games (especially if you take advantage of steam and other digital distribution deals), and tons of mods. I've spent less than $150 on games, and I have about 20 of them, not including the 5 or so Half Life 2 and Unreal Engine mods I've played. Console games, on the other hand, rarely dip below $60 apiece, and their replay value is totally controlled by the developer. Recently we had IW release 3 new maps for $15, which MW2 fans happily paid for because they're both idiots and have no other way of prolonging the life of the game. That would be unacceptable on the PC, seeing as there are tons of mappers and modders around, all creating new game modes and maps. If a game picks up enough modders, it won't need an expensive sequel like consoles would for a very long time. Starcraft, a game made 12 years ago, is only JUST getting a sequel now. And it's STILL being played today, and will be likely still be played for a very long time. Why? Because of the fantastic things the modders have done with it.
So, entry price can be very low depending on situation, games are cheaper, have longer lifespans prolonging the time between sequels, DLC is free, and multiplayer is always free (except in the MMO scene). It starts to look really easy on the wallet. And it is. I spend so little on gaming these days.