Much depends on the condition. I rescued a Windsor No.1 tenor banjo from a junk shop about 25 years ago for about £80, and spent a fair amount having it upgraded to make it reasonably playable. However, I suspect it would be unlikely to fetch more than £100 at today’s prices, because of the easier access to decent vintage instruments via the Internet now at competitive prices. Nevertheless, an old Windsor is likely to be much better than the typical cheaply made banjos you find in non-specialist music shops. And yes, I have vintage mandolin banjo, not a Windsor, that someone gave me, but it is not just barely playable but untunable.