Your quoted prices are not realistic for verified, working hardware. The motherboard is also definately not negligiable. You can't spend less than ~$50 on a new motherboard and if you want a used one, chances are you will end up spending more than that $50 throwing out the trashed ones and looking again. A working, used motherboard is worth at least 30 bucks as long as it supports a chipset that is at least semi modern.
CPUs are easy to find, but you'll spend more than $10 after shipping (and good luck finding something specific locally).
RAM has to match your motherboard specs so while someone like me (and you perhaps) has enough lying around to match most systems, you'll have a hard time paying less than $20 for any working DIMM and the benefit of buying the motherboard, cpu, and memory all together isn't negligible.
Video cards and hard drives on the other hand, I do think are negligible. Both are very swappable and easy to find. I have 3-4 spare IDE drives sitting around ranging from 30-120 gigs each....all of them I have received for free and are in working condition. Video cards generally aren't even necessary since most people are content with integrated graphics (virtually standard).
I'd say no less than $75 for motherboard+cpu+ram, the rest is arguable and likely much more valuable as a part of the 'working' whole than individually.
