Believe it or not it's not for the silver, it's because of these things:
1) The Mac has hardware in it the PC does not, Apple motherboard etc.
2) The Mac can run OSX, the PC cannot (not properly)
3) The Mac has been upgraded using expensive apple branded RAM etc, which IRL people shouldn't bother doing, the PC is a custom build using better value parts.
4) Apple RAID Cards are retardedly expensive, it's about $700
5) the 5870 is designed to run with an EFI, again making it more expensive.
6) The monitors used are not comparable, just because they are the same size, it's like saying a ford fiesta and an Aston Martin are comparable because they have the same number of doors.
7) What Xeon processors? You can get Xeon Processors for £10 or £2,000 depending on which ones.
8) Apple cases cost more.
I think that covers everything but I summed it all up with this statement:
Ones a Mac.
OH! And except for the RAID card my Mac Pro has all of that and cost me.... £2,500 or so