midi keyboards can range from as cheap as $100+ to probably $1000 or more. they have different features and all, so you want to have a balance between what you can afford, and what you want in a keyboard.
if you just want it as a midi controller and u don't need other features like sound patches, midi triggers, knobs etc, then it wouldn't be that expensive.
1 thing u might want is weighted keys, and being able to pitch shift and bend =]
after that you would need a sound interface. something that has a midi in input. and then u're good to go =] this is recording in midi/signals mode mind u. u can always get a keyboard with its own sound effects and record audio rather than midi (midi keyboards traditionally have no sound effects of their own)
hope that helps =]