If I thought the frame was salvageable, and I wanted this bike as a rider, I'd pay $100. If the the rust is cosmetic and the frame and fork straight, putting a $150 into paint or powder coat , would get you a $250 frame for $250. If you put another $150 in appropriate components and consumables, you might have $400 into a bike worth $500. I have a 728 in the garage that is in better shape, with original components, Bruce Gordon Racks and panniers ect,ect, but its same economy, not being worth much, except I could just continue adding to the 20 years of nail polish touch up and ride it around. Your bike is raw project material, and realy needs paint, so unless you want a project....
You could abrade and neutralize the rust and rattle can the bike primer black and ride it like you stole it, but thats a lot of work for a halfway solution.