High quality PCs certainly exist, although you won't find them at Best Buy (a well stocked micro center or fry's may have some, and they're mostly sold online). You'll pay at least as much for a well built PC a you will for a comparable Mac, and somewhat more in some cases. My father has a MacBook Air, and my mother has a Toshiba of about the same price, that is also a thin and light machine of similar specs (it's about 1/2 lb heavier, but has an optical drive). There is no comparison - the MBA is a much better built machine. The Lenovo that is substantially more expensive than the MBA is at least as well made, though ( and some of the higher end Sony VAIOs are apparently gorgeous, although more in the price range of an RMBP than an MBA). Macs are competitively priced against their real competitors (EliteBooks, ThinkPads, Dell Precisions and upper end XPSs, the nicer VAIOs, etc...).
Where Apple gets in trouble is that there are several computers they choose not to make, which people want to buy (and wish they made). Here are at least some of the "often desired Macs" that Apple resolutely refuses to introduce.
1.) any cheap, commodity computer - there is often a Mac Mini that almost counts, but there's never much choice.
2.) any cheap laptop (other than an iPad). If you want a sub $1000 portable from Apple, it'll run iOS, not OS X. I'd rather use an iPad than a $500 notebook, but there ARE things a $500 notebook can do that an iPad can't.
3.)any expandable desktop other than a $4000 workstation. This one is my personal bugbear...
4.)any computer built for gaming (notebook or desktop).