Radeon 9600 Pro: $47 (http://www.newegg.com/Product/Product.aspx?Item=N82E16814102410)
GeForce 6200 LE: $52 (http://www.newegg.com/Product/Product.aspx?Item=N82E16814130014)

Lots more if you have a PCI Express slot...

Oh come on... I'm playing on a lappy with a cheap onboard Intel graphics chipset, which is vastly inferior to the GeForce 6-series, and the game is still perfectly playable for me... as long as I stay out of ion storms. In ion storms, my framerate drops to 10 FPS every time, whether my settings are on max or min, it makes no difference.