It's doable, but yeah I agree the market for build-it-yourself Macs is pretty small.
I was talking about the upgrade versions of both OSs though. Right now (according to Apple.com) OS X Leopard costs $129. On TigerDirect there are several upgrade copies of Vista SP1 ranging from $103 to $230.
So yeah, Vista Home Basic is cheaper than OS X, but I'd argue that OS X comes with more than $30 worth of software to make up the difference.
I do think that Apple hardware is horrendously overpriced, and I think maybe that's what you meant, but that doesn't really belong in a discussion about the pros and cons of each OS.
No, but I'm sure they'd love donations.