There was an article about budget cards in Computer Gaming World magazine, here's some ideas for you:
All tests are at 1024x768, no AA or AF on a 3.4gHz Pentium 4 EE system, 1GB RAM.
Under $100:
GeForce 6200TC 64/256MB - 1358 3DMark's (2005), 18.4 Doom 3 fps.
Radeon X300 128MB - 1432 3DMark's (2005), 18.7 Doom 3 fps
$100-$150
Geforce 6600 256MB - 2108 3DMark's (2005), 51.1 Doom 3 fps
Radeon X600XT 128MB - 2108 3DMark's (2005), 29.9 Doom 3 fps
Radeon X700Pro 256MB - 3319 3DMark's (2005), 54.1 Doom 3 fps
$150-$200
Geforce 6600GT 128MB - 3826 3DMark's (2005), 83.6 Doom 3 fps
Radeon X800 128MB - 3733 3DMark's (2005), 71.1 Doom 3 fps
Over $200
Geforce 6600GT OC 128MB - 3826 3DMark's (2005), 85.9 Doom 3 fps
Geforce 6800 256MB - 3992 3DMark's (2005), 93.9 Doom 3 fps
Radeon X800XL 256MB - 5038 3DMark's (2005), 89.5 Doom 3 fps
The prices may have changed since the article was written, but it gives you an example of what your looking at for what price range. There is a giant leap in performance from the under $100 and the $100-$150 range