at least pick another workstation to compare to the Mac Pro...
take a look at a real HP workstation, the pricing is similar to that of the mac pro if you take hardware differences into account.
http://www1.hp.com/
now try and build a Lenovo workstation similar to the Mac Pro
http://shop.lenovo.com/
again, the Mac Pro isn't overpriced compared to the Lenovo D:
The Mac Pro isn't comparable to the HP Z800.
Mac Pro :
8 Memory Slots, 64GB Max (Dual-Proc MP only)
1 PCI-E 2.0 x16, 2 PCI-E 2.0 X4
Radeon 5870 maximum video card from Apple, dual-GPU limited to two 5770 (PSU and qualified video cards massively gimped on apple side)
4 Hard Drive bays, limited to 6 hdds even if you order the optional SAS Raid Card
Z800 :
12 Memory Slots, 192GB Max
2 PCI-E 2.0 x16, 2 PCI-E 2.0 X8
Supports crazy number of video cards, up to 6GB Quadro 6000s
Dual GBIT Lan, Infineon TPM
6 Hard Drive bays including 4 SAS Hot-Swap bays, 14 total Sata+SAS connectors
They're just in a different league. The MP is sort of in the middle, offering more max ram but weaker video and almost everything else than the generic HP Elite Business PC above even when taken to max in single-cpu form. In dual-CPU setup (8 ram socket mobo), the MP slots in pretty heavily below the Z800 series.
EDIT : Checked out the Lenovo workstations, yeah they seem terribly priced and not that impressive in expansion/etc. Does support dual 2.5GB Nvidia 5000s, but for the most part seems weaker than Z800. That's all exotic territory though, it remains a general truth that the 'workstation' label is just an excuse to bend people over with the pricing, whether it be Apple, HP, IBM, or otherwise.