"What is it worth?" and "what should I pay?" are common questions that no one can really answer. It depends on so many different considerations, that it really comes down to the question of what it's worth to you. Rather than looking at what others are asking on eBay or Craigslist, I like to look at the history of sales on the Shop Goodwill site to see what others actually have paid. Here's a link to all the sales of machines described as Featherweights for the last 30 days.
http://www.shopgoodwill.com/search/S...&showthumbs=on