I have a Featherweight that was my Mum's. She bought it 20 years ago for $125 (used).
There is an elderly gentleman here in town who used to work for Singer many years ago. He still does repairs in his home and I have taken all my old Singer machines to him for servicing. I asked him about the Featherweights once and he said they are overpriced and that if you can get one for $300 then that should be the maximum one should pay. Just FYI.