Thought I'd share this!
I picked up a Yamaha DX7 on Ebay about a month and a half ago for... ?27 (about US$54).
OK, it's not exactly in pristine cosmetic condition but it's fully working and guess what, it has a very expensive memory upgrade (Sycologic V2) which I didn't know about, which gives 4 banks of 32 sounds (instead of a single bank), and it has 16 MIDI Out channels (the DX7 only transmits on Ch. 1 normally). It can send Program Change messages 0-127, whereas the the basic model only sends 0-31. No Local On/Off though. The only annoying thing with the Mark 1 DX7 keyboard is that it does not transmit velocities beyond 118 so beware if you're thinking of getting one to use as a MIDI controller.
I just love the DX sound; I also have a TX7 expander and once owned a DX7S, which I stupidly sold.
Gilles