Can you give a little more specifics on what you want to do with it?
mAudio makes a set of excellent "keyboards" for under $100. I say "keyboard" because although they are pretty much just passive MIDI interfaces. Plug in the "keyboard" to the USB, and the "keyboard" relies on the onboard synth of the Sound Card to play the audio. However, this would mean that to use it you record or play need it plugged into your system. In this case, you need to rely on software (Sonar, Cakewalk, etc. for arrangements).
If you want a board that is a little more, you can try the Yamaha PSR Series. Either the E-303 or the PSR 295 is good. Difference is about $40. PSR 295 is better IMO though.