Define cheap. For about 1000 Euro or a little less you can get a Linnstrument 128 which has din-midi
The qunexus keyboards (keith mc millen) have din midi (via adapter) and a kind of MPE mode, no pitchshifting/sliding from one to the other key, but pressure and position sensors. Those are much cheaper (around 150 i guess)
But it should be easy enough to program an arduino with an usb host shield (or a teensy) to send usb-midi to serial midi output....