On average, if you can FIND one, about $700 is right. A little more wouldn't be out of line if its in really good shape; a little less would be welcome, obviously. Be aware that it has no disk drive, only card slots.
I'd love to land one myself, as it would really make my 0/1Wfd sit up & bark, what with the total 64-voice polyphony it would confer. That would allow you to dump one sound set to it and have another resident in the fd. Bitchin'!
Also, I believe it only has a 7,000-note limit in its sequencer, as is the case with the basic 0/1W, so keep that in mind.
I hate to potentially harelip myself in locating one someday, but you should regularly scour
www.ugbm.com and
www.roguemusic.com for 2nd-hand anything.
Also,
www.musichotbid.com is a new music gear auction site being advertised in Keyboard magazine. Haven't checked it out yet, but you might want to do so. Can't hurt.
Good luck!