You want a SYNTH? Or just a digi-piano?
As long as you go used, there should be plenty of digi-piano's around. Check out Roland's FP series. nice sound and action. Kurz
SP-88 action is crap, but you might find a used 88x (way better action) for $600. The "award winning" sounds aren't too great, though. And Kawai's blowing out their 9000s now that the 9500 is out (I think those are the model #s), so that might push the used ones down some more.
For a full synth with weighted keys, good action, and good piano sounds, GFL. Maybe a used
Alesis QS-8? Piano sounds are barely adequate, but action is great. Ensoniq KT-88s were pretty good, don't know going price for those.
Last thought - weighted controller with a synth or piano module. Yam P-50s and Roland P-55 piano modules should be way cheap. A used Fatar controller plus a P-50 might get you close to $600. But a cheap synth module with GOOD piano sounds??? Dunno.
