And you need to buy CPU, Board, HD, RAM, case and monitor from that?
There you go
Seriously, there's almost no way, even if you cheap out on everything.
What is the purpose of this system?
2.13 Celeron D - $51
That's the heart of the system, you can get a board for probably around $50-70, looking at about $50-70 for about 512 of PC3200 RAM, $50 for a 40GB hard drive, a cheap no frills case for about $40, that's $261 roughly, then add in a monitor for about $100 for a CRT, and there's your budget right there.
Now, go to Dell.com and build one of their sale PCs, check the specs against that set up and you'll see more bang for your buck.
Building your own PC will not usually be cheaper than Dell or gateway, ect, even if you use the cheapest componants. The cost benefit to building your own is to be able to upgrade later and to have better parts that will last.........