Well, was I ever high in my estimates! Just looked over the last sold banjos on eBay, and a 1920s TB-3 only brough $850! I AM surprised. And think maybe the eBay prices are a little under what collectors would pay. Who knows?
Try sending a note to Mike Holmes at mugwumps.com (look for his website) and ask him.
I couldn't open any eBay items themselves, unfortunately, so I don't know the conditions of those being sold. You might also try vintage guitar magazine's web page (again, I don't have the URL -- I changed PCs recently, and the old one has all my "favorites." Owing to the non-running of that machine (the reason I replaced it) I haven't gotten to my data yet.