If you're really low on budget, go out and buy one of those home-keyboard things like what yamaha and casio make that have a midi output. You can get a decent 41 key one for under $100, and they have midi in/out to boot.
Drawbacks would be they are cheap, therefore the weighting on the notes is normally far off, not all of them can do touch sensitivity or have foot pedal inputs, and most importantly they are a lot bigger than most midi controllers, so you would need them out the way, possibly on a stand.