You based that affirmation on what? I just went to the Dell website (which I can assume are the cheapest PCs out there) and build a 13' Laptop with mostly the same specs as the Blackbook. It was 1169$. Then went to HP, it was 1,153.99. And it's not including Vista Ultimate (the nearest version of vista in comparition with OS X) Anti-virus softwares (which are not needed on Macs) Microsoft Office, etc. And accounting the Blackbook isn't even needed, we can take the mid-range Macbook and upgrade it to 250GB, which is really only about 250$ extra for a brand new Macbook.
Apple charging double is not totally true. On MBP & BTO options I can't argue. But base models aren't that expensives.