There's a wide range of stuff available used in that price range, I'm sure. New, the only things approaching that kind of price are the Alesis QS6.2 and the E-MU range. I have 2 E-MUs (a PK-6 and a Vintage Keys) and I like them a lot (much more than the Triton Classic or the Motif, which I didn't get on with the action of at all) but a lot of people diss them. Certainly they look a bit cheap, they're very plastic, but they're reliable and use the well-established Proteus 2000 architecture, they're plenty expandable (I have 3 ROM cards out of a possible 4 in each) and E-MU are blowing them out at the moment.
I don't know what's going on at E-MU (probably Creative applying the squeeze) but their products are getting cheaper all the time. I remember when the PK6 retailed at Ģ999/$1195... now you can find it quite easily for Ģ299/$399. Expamsion ROMs are pretty cheap too, direct from E-MU, having come down by as much as 70% in recent months. The presets on the PK-6 are quite lame (the Halo and the Vintage Keys are much better) but it's a powerful and easy-to-use engine that encourages you to edit and create much better sounds (it's very possible) and you can get great results from it.
I'm not trying to be an E-MU evangelist here, just that everyone talks about Korg, Kurzweil, Yamaha and Roland (occasionally Alesis) and forgets about E-MU... which is a damn shame, you get a lot for your money IMO :)