Only "rare" 308 featherweights are the carbine models, they have a 20" barrel if I remember right. There are obviously other rare M70s, but generally are in different calibers and configurations.
Well used can be a lot of things. I wouldn't pay more than $900 if it was decent, the bottom value seems to be about $6-700 pending rust and bore. I window shop them regularly, keep thinking I need another one or two, but my money hasn't jumped out of my wallet yet.
As an FYI, if this is for the kids... take the aluminum but plate off and put a real recoil pad on it if it still has it. They are not fun to shoot as is.