Just my non-expert opinion, but I think you should be able to get $700-800 for any trike with a good frame, good rubber, and 'new enough' drive train. Seven and eight speed cassettes are probably a liability, as are worn parts (especially chain.) Most trikes have disc brakes.
OTOH, if you've got something with an out-of-spec chain and worn cassette and chainrings, you could easily end up spending half its value just to fix it so it'll sell.