Loads of keyboards these days do midi over USB. I've got a £100 Casio job that does it. Can use it as a MIDI controller with my laptop and can also take songs and voices across from the laptop onto the keyboard.
It's rather large though, but I'm sure you can get smaller and much cheaper ones just to act as midi controllers.
soundslive.co.uk has a whole category for them:
USB Midi Controller Keyboards, the best deals on-line