Apple used to offer more options. At some point (in the late 90s), they decided that the number of choices was too confusing, and they would have better sales with a smaller number of easily differentiated products, and better marketing to convince you that one of them was the one you wanted.
Of course, the more cynical view is that this lets them convince you to buy something more expensive than you might otherwise need in order to get certain features, like some cars require you to buy the more expensive trim package just to get anti-lock brakes.
Regardless, part of it is also the premium on Apple hardware -- even for a system fairly comparable to one from another manufacturer, you're going to pay more, which is going to pay for the design, engineering, and marketing.
If you're really on a budget, your best bet is to buy a decent PC laptop from a company that won't force you to buy Windows with it, and put Linux on it (probably Ubuntu). But Apple has the advantage that it Just Works right out of the box on a lot of levels, and most people don't want to deal with futzing with their operating system just to get sound working, for example. (Ubuntu is actually getting pretty good in this respect, but it's not all the way there yet. Apple has a huge advantage here, in only having to test a limited number of known configurations.)