To be fair Apple has to assemble the parts itself which takes some money. But a Mac in a lot of configurations can still run hundreds of dollars more than computers built by Dell or any of the large pc makers.
A typical defense from the Apple side is that you are paying for quality and that Macintosh's fail less. Well let's take a look at the data:
Apple has virtually the same failure rates as Dell. It makes perfect sense; they get all their parts from the same companies. Dell and Apple get their processors from Intel, they both choose the lowest quality hard drives such as Western Digital's green line, and even their motherboards both come from Foxconn, which is regarded as one of the worst pc parts manufacturers. For both companies, support for low tier products is 1 year, support for premium products is 3 years. Apple's "high quality" is a myth created by their image and their marketing.
You can argue that OSX's interface is easier to use and more productive or whatever. But interface really comes down to personal preference (and not to mention you can install OSX on a PC). Stability for both operating systems has been outstanding since Windows XP and OSX. The only real difference between the two is that Windows has 100X the amount of applications OSX has.
So if a Mac gets its parts from the same places a Dell does and has virtually the same failure rates, what are you paying for? You are paying for a Dell… in a shiny plastic box.
To see the other side of the argument from an Apple fanboy take a look at my coworker's blog here.