I'd pay up to $100 and then either ride it or part it out. The quill seatpost is worth something, as is the Stronglight crank and the derailleurs. The fork too, if unrusted.
If I were keeping it to ride, I would fill the dent and touch up paint. With model (hobby) paint you might be able to mix up a decent match. The dent is just too ugly to leave alone.