This is a very common and annoyingly inaccurate misconception. Apple's laptops are not expensive at all, and are infact priced very competitively. The reason they appear to be so expensive, is because they offer a new range of high-end computers, and a solid mid-ranged class of computers, with absolutely no low-end computers.
Apple isn't going to sell a computer unless it can function correctly without crashing or lagging by performing simple tasks, unlike Dell and other PC prebuilder's low-end models such as HP and Gateway.
You can buy that laptop from Gateway, but the CPU is gauranteed to be literally at least 1/2-3/4 slower than the iBook, and it's going to have 256mb of memory vs. the iBooks 512mb standard, not nearly enough to run Windows XP without lag. Not to mention Apple's laptops have physical video cards, not integrated chips like 99% of the low-mid ranged laptop market.
Also, Mac's can run Mac OSX and Windows XP (both officially supported by Apple) where as PC's can only run Mac OSX.