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Remember most people simply tend to think of hardware vs hardware. If, however, you add the software stack necessary to put the two machines on par then the price difference shrinks incredibly (not to mention the fact the the Mac software integrates much better than the PC software). If, on top of that, you add the extra time it takes to administer the boxes, many people would assert that the value (or TCO if you will) is much better for the Mac.
So, to reiterate Thom's point, whether a Mac is expensive or not is very subjective.