So I figured for some actual numbers:
BTO a Mac Pro off Apple's site, a 6-core 3.5GHz Xeon with 32GB of RAM, dual D500 GPUs, a 512GB PCIe SSD and AppleCare (to match HP's built in 3 year warranty) comes to $4948.
BTO a 620 on HP's site, and doing the same specs with a pair of W7000s takes you to $6609. Sadly HP doesn't offer the precise equivalent to Apple's cards, but finelet's just knock it down to a pair of FirePro V3900s1GB each compared to the 3GB D500s. You're still paying $5019.
(I tried configuring a Dell Precision but they don't have the same 6-core E5 Xeon I used in the comparison above. Using a 2.6 6C and a 3.5 8C I got between $4200 and $5300 using dual W5000s, which are still worse than the D500s.)
So there's lots of rebuttals to thisyou have more than (currently) three graphics cards options on the PC, you have more slots for more RAM down the line, as well as more HDDs. And all of those are a valid, and I can see use cases where that sort of internal expansion is essential. On the other hand you're getting faster storage, quieter, more power-efficient and space-saving design with a Mac that will hold its resale value tremendously (I recently sold my 2008 Mac Pro for $1000, and my 2008 Macbook Pro for $500, complete with a nonfunctioning optical drive.) But to argue that Macs aren't competitively priced, especially from a workstation standpoint, is deeply misinformed.