I have an 83 500 that I have built up from a frame using mostly new parts, ie, wheels, tires, deraileurs, 7 speed cassette, seat, stem. The rest of parts are recycled and I have about $400 in it, a third of what a 520 is new and it is set up just the way I want it. What it is worth is not important because I didn't build it to sell.
If you have nothing invested whatever you get when selling is a plus. I am sure someone would be happy to have it.