As far as I am concerned, the "you get what you pay for" principle applies here.
I have both a good gaming PC and a 360, and since I like playing games from the couch they should compete with each other for that sacred spot near my TV set. The end result: my 360 remains buried underneath a book pile in the corner of my living room for, what now?, six months?
Since I completed Gears of War and Dead Rising, I found out that every single game simply looks prettier on my PC, which I got for ~$1200. I manage to run most games in full 1080p resolution on that thing, with nearly everything maxed out.
So, are gaming PCs more expensive? Yes, they are. Are they worth paying the extra? Absolutely.