Hardware pricing.. hopefully the last word
It depends what you're comparing and what your needs are.
If a Mac Pro is compared to an identically configured PC, it comes out very well (because it *is* a PC, with a few unusual features such as EFI). Watercooling, nice case, Xeon processors, large memory support etc.
Even if you pick a high end non Xeon system with a decent case (100UKP+ instead of a shitty 50 quid job), a proper crossfire/SLI full speed motherboard etc, the difference still isn't that great.
The real problem is when you compare specific functionality rather than pricing. So, if your requirement is 'quad core system with one fast graphics card' then the PC system will be as cheap as a third of the price of a Mac Pro. It won't have the upgradeability to two processors (8 cores+), but it will achieve the desired functionality, and that's where Apple falls down. So far they haven't addressed the midi tower market, probably because it would cannibalise Mac Pro sales.
It would also help if they revamped the mac mini, which has an embarassingly old graphics chipset (even from a mac perspective) and lacks any multimonitor support without hacks like a Matrox Dualhead2Go.
So, really, whether a mac is 'expensive' depends on your viewpoint. Don't believe me? Go look up part lists and you'll find I'm right.
Still, you can't argue that Apple haven't innovated and been rather successful, even if you don't agree with their product implementations or buy into the target market. No, I don't have a Mac - I may consider it if they revamp the mini. I can't stand integrated computers and justifying 1.5->2K of new PC is years off.