The prices really aren't that out of line with the options that come on that beast.... A PC specced with the exact same components probably wouldn't differ more than a few hundred dollars if purchased from say, Dell. And the video card support for OS X is a bit lacking, therefor the offering's aren't exactly tops. I think in the near future we'll see DX10 cards fully compatible with OS X, 8800GTX/R600 8-core Mac Pro's FTW!
Asus Sabertooth 990FX | AMD Phenom II X6 1090T | 16gb G.Skill Ripjaws X DDR3 | Thermalright Silver Arrow
2 x Sapphire HD 6870 1gb CrossfireX | LG 10X Blu-Ray Rewriter | PCP&C Silencer 750 Quad | Fractal Design R3 Case
60Gb Patriot Pyro SATA III SSD | 1Tb WD Black | 3 x 2Tb WD Green | 2 x 360Gb Seagate Barracuda