"How much lower" who can say. My friend runs a one man business building computers. His lowest priced is £230 but only has a 40gb hard drive.
As you say, you can't buy the parts unless you are a bulk buyer.
I also remember my first computer, an IBM with no hard drive at all, only two floppy 5.25 drives, 640kb memory. I wouldn't like to say what the processor speed was and it cost me £400 and that was secondhand.