Can I ask where you're located? I have an '82 Trek 613 that is in very nice shape, but I've been wanting to improve it with lower gearing for some light touring (I only have a double chainring). I've been looking for a 520/620/720 that I could maybe swap in the appropriate bits (derailleurs, BB, crank, freewheel) to effective turn my bike into a 620.

BTW, I paid $275 for my bike a couple years ago in basically NOS condition. In my area, a 620 in good shape would run about $300-350. One in rougher shape that needed some work would go for about $200 maybe.