I have watched auction shows where they have gone fairly low. So low in fact I just wanted to scream. So hope that happens to you. And if there are dealers in the audience they will stop at 50% of the value because they will want to make money. So if you can verify the age (which you can do with just the serial number) and verify that it works, sounds good, is clean, I would say not to go over $300.00. There are too many on ebay and Shopgoodwill.com to pay much more than that.
But I don't know what your area is economically. In southern CA they go fairly high in the quilt shops and dealers. By that I mean $500.00 and more. Of course they come with a warranty and have been serviced recently.
The featherweights are easy to work on, because they are the most basic of all sewing machines. The motor can go out, and some have to be re-wired. But not real costly from what I have heard, some say their husband does it for them but I can't talk to that. To replace the foot pedal is about $50.00.
You might want to go on line and look at the manual to get an idea of just how easy they are to oil and lube. Also you can see what should come with it.
I love mine, haven't had time to really play with mine. But I was able to get all three up and running perfectly within minutes even though when I got them they were misbehaving.
Wish you luck because you will love it.