I don't know if you can find one used for $100, but an ATi X1950 based card would be near the best in that ball park price range. I'm not sure about NVidia offerings, but others can possibly give you different recommendations.
Unfortunately for AGP you don't have many choices. Your best bet is probably like the above poster said, try and find a 1950 used. For a new card, you're limited to something like an ATI Radeon HD 2600 or a GeForce 7600 GS.