The KAMI-OTO is a fun and easy to build DIY cardboard keyboard, it is velocity sensitive and can be bought on their Kickstarter in either Bluetooth or Micro USB for around $28.
If you want a cheap MIDI keyboard then this may well be the one for you!
More info on their Kickstarter!
And viewer questions
Zebra 2.9 + ZebraHZ and 4000 or so patches
Gets an update
Macro Controls and viewer questions