CPU, Motherboard, RAM... (Prices vary, but were pulled from www.newegg.com)
Athlon II x2 240 - $58~
Asus 780L motherboard -$65~
2Gb of DDR3 RAM - $50~
Total: $170~
(Try to get 1333Mhz or 1600Mhz from any company that isn't Kingston or a no-name RAM company. A-Data, G. Skill, GEIL, Corsair...