Let's talk real numbers. A loaded 12 core Mac pro costs about 12.5k. 2 2.93 Westmeres, 48gb ECC ram, 3 2 TB Drives, 512gb ssd, 2x 5770s.
The exact same hardware costs $6038 total. So you're literally paying double the cost. Newegg for proof:
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So that's for the exact same industrial grade hardware. It's double the cost.
If you are willing to lose ECC, you can have the PC hardware cost yet again since you're not stuck with Xeons.
If you're willing to keep equivalent performance while changing componets, you can save tons of money again. For example, use a single video card (higher overall performance), 3tb drives, etc.
I'm not going to argue necessarily that it's overpriced, as to some people it is worth it to pay double simply to have the Mac logo on it. If it's worth it to you, it's worth it to you - if people knew what I spend on clothes, they would have me locked up.
That being said, just be aware that on a top of the line Mac Pro you're paying about double RETAIL prices for the hardware - never mind the volume discounts that Apple has.