A Fatar SL-990 is probably about the cheapest 88 key controller you can find. It has a great weighted hammer action and is only $499 new so you could probably find one cheaper used. If you want aftertouch and more onboard control then go for the Fatar SL-1100

But you will have to buy a MIDI control surface for extra sliders & knobs. You won't find slider/knob controls plus 76+ keys for under $400.