There is nothing, other than a unique design, that makes a Mac worth more than a PC. In fact, if you take the price you pay for a Mac and put it into a PC you'd end up with a much faster machine, hardware wise.
The manufacturer for Apple mainboards is Asus, just like it is for Dell, HP, Compaq, etc. (for the most part anyway, I know there are exceptions)
I think you're mostly paying for the funky design, much like you pay more for a laptop than you do an equivalent desktop.