In perfect condition, I'd pay absolutely no more than $50 (if I needed another copy). Most copies will probably go for between $10 and $30. It's a great program, but whoever is charging hundreds of dollars for it is nuts (and whoever is buying it at that price is completely bonkers!)
I'd suggest holding onto your copy and enjoying it. As someone who can't sight-read music at any speed good enough to be useful, DMusic has been quite valuable to me in helping me learn tunes and playing with arrangements.
