I think $100 is a reasonable possibility for a Reynolds 531 frame. It all depends on the quality of the frame. A lower end 531 frame would be something like a 70s Raleigh Supercourse. I have seen them go for around $100 on ebay. However, a Raleigh Team Pro made from Reynolds 531 would go for much higher. The stamp of 531 is definitely a stamp of quality but some are definitely better than others.