It's supply and demand - plain and simple.
You put a FREE Featherweight on this board, and see how many people contact you for it
Put a FREE 66 on this board - and you may not get any.
Now the real craziness is with attachments. As most people on this board know, with one or two expections - if it fits a 66 (excluding back clamp) It's going to fit a FW!! and all the Singers in between . . .
Yet people with Featherweights will pay 3X as much for the exact same item if the Seller shows it attached to a FW instead of a 66.
I bought a FW because I'm a Singer fan- and it was one of the most expensive Singers I've bought - way overpriced if I compared it to something like a mint condition 1895 28 in a Gothic style case with an original hand crank.
If the Quilt Clubs get into a machine, then the price is going to go up, as there are a lot more Quilters out there than Singer machine collectors.