Really? This argument again?
I built an i7-based system for my gaming need, and here were the parts:
Intel i7 920
6 GB Corsair DDR3 1600
ASUS Rampage II Gene motherboard
Lite On DVD+/- RW drive (SATA)
One 640GB HD (SATA)
Radeon 5850
Corsair HX750 PSU
Corsair H50 CPU water cooler
Linksys WMP300N wireless adapter
mATX case
Logitech Illuminated Keyboard
Microsoft Sidewinder X8
Windows 7 Professional
Total system cost? $1545 (I actually got Windows 7 Pro for $40 through the educational discount, but the price I used for the estimation was the $130 OEM cost I've seen around)
The entry-level Mac Pro uses the server-based version of the i7 920, but it's comparable. Here's how I spec'd the Mac Pro using Apple's store:
Xeon 2.66 Ghz
6 GB Ram
640 GB drive (default)
ATI Radeon 4870 (last-gen, 5850 is easily better, but highest-available option)
18x SuperDrive (default)
AirPort Xtreme Wi-Fi card
Standard Apple mouse, keyboard
Cost? $2899
So comparable systems roughly (I'd argue the system I built is better, due to a likely better motherboard with SLI/Crossfire support, and the Radeon 5850), but a ~ $1300 price difference. So you're essentially paying $1300 for the Apple name, case, and labor.
I also checked Dell, and after suffering through their horrendous selection process, I was able to similarly spec a system (except using a 5870 instead of a 5850) for $1799. A couple of hundred more than mine, but still $1100 beneath Apple's price.
Don't get me wrong, I love OS X, and think one of it's best advantages is the close-knit nature Apple maintains between the hardware they develop and OS X. But don't try and say that Apple's prices are the same as others.