MB -
http://www.newegg.com/Product/Produc...82E16813138332 $50
CPU -
http://www.newegg.com/Product/Produc...82E16819116397 $88
HDD -
http://www.newegg.com/Product/Produc...82E16822136108 $60 - these mechanical drives are going to throw a wrench into your budget right now, this one is the absolute cheapest piece of junk I could find that conforms to serial ata.
Case -
http://www.newegg.com/Product/Produc...82E16811154109 $20
Memory -
http://www.newegg.com/Product/Produc...82E16820231308 $35 (went big cuz I think we have a surplus in the budget - this is 8GB)
PSU -
http://www.newegg.com/Product/Produc...82E16817170010 $19
That's all you need for the base of a computer - total is $272
If u want win 7 - probably looking at another $80 for home version, then allow about $20-40 on top of that for keyboard/mouse/headset/speakers (if needed)
and allow another $50-100 for monitor (if needed)
That harddrive is hit or miss on using as boot device imo, but was cheapest I could find.
core components sub $300, mission accomplished.
there's ways to get cheap windows.