When buying a synth keyboard - or a module with a MIDI controller (ie: keyboard) you have to determine what your needs are.
Some people have mainly piano chops - so they want the keyboard to feel and respond like a piano (ie: weighted keys) which can add much cost to a synth.
However, a stand alone controller (no on board sounds) with weighted action can be much less costly.
When considering synth functions - it often is mostly about sounds (realistic piano, phat analog, etc) and the next considerations are functions (easy to program, how many sounds at once, etc.)
I'm a fan of a keyboard controller with some on board sounds - and then modules on top of that. At least that way, the keyboard can function as a stand alone (say - for a gig), or if your MIDI routing (say -computer software) fails - you still can create music.