There's been a PX 10 for sale for around $600 not too far from me for over 2 months with no movement. It's overpriced even though the bike is in a decent sized city. $400 to $500 is probably about right in most decent markets, less in smaller cities. The stronglight 93, even french threaded, is worth some coin, as is the saddle.

You threaded the drop out to take the shimano derailleur. That's actually a very useful thing to do but that will hurt the value as well. Do you still have the original simplex rear derailleur?