Alot of the cost goes in quality and feel. Cheap keyboards will have a cheap plastic feel, and sometimes arent velocity and wont have aftertouch. more exspensive will get you hammer action, velocity, aftertouch and so on. It all depends on your needs. I think you will be happy with synth touch and velocity, such as http://www.musiciansfriend.com/srs7/g=key/s=midi/search/detail/base_pid/709204/

For high budget and want pretty much the best, something like http://www.musiciansfriend.com/srs7/g=key/s=midi/search/detail/base_pid/709203/ will be in your field.

Phil