They're not, actually.
Apple doesn't address the bottom-feed low end of the personal computer market place. If you compare similar technical specs, including things like how well the hardware is made (quality and value-retention), and how much you have to spend on software to get a robust and reasonably secure system, Macs are generally LESS expensive than the equivalent Win-PC.
But because you -can- buy a cheaper (both quality and cost) Win-PC in the low-end market where Apple doesn't compete, the perception is that Macs are more expensive.
Having had literally scores of personal computers over the years (started in the late 70's), including home-brew, Apple, IBM-PC, clones, Macs, loads of Win-PCs, Linux, Unix, etc., I can tell you that Macs deliver better value than a Win-PC, all other things equal.
The absolute highest value for the dollar is probably Linux on home-brew generic Intel hardware, but that's not everybody's cup of tea.
That’s probably why I could put a machine together with just the things I need for under $500 on CyberPowerPC.
Apple does have a low-end line it’s called the mini.
I agree with your point about the best price to performance ratio is to purchase a core 2 duo or a phenom2 box and install your favorite flavor of linux.