Case: $69.30
Motherboard: $100.10
Processor: $118.30
Memory: $77.90
Keyboard: $19.10
Mouse: $13.60
HDD: $78.90
CD: $54.40
Monitor: $178.90
---------------------------
Total: $710.50

I'm going to disagree with bouy_wonder - while you might be able to put together a really basic box for $400, I don't think you could include a monitor with that and it sure as hell wouldn't be any PC I'd want to use. :)