Slipjoints have the largest growth of any part of my collection, based mostly on bang for your buck. A couple of weekends ago I picked up 3 knives, each around 100 years old, in pearl and stag (2). The pearl was $2.50, each of the stag were $10. Great users, some collectibility, and fantastic value for the buck. From flea markets/antique shows or stores I rarely pay more than $15 per knife. They won't appreciate in value, but I will appreciate them!!
P.S. I hope you have made your way to the outdoors forums. There knives are talked about in another traditional way- talking about using them to cut stuff!! Have fun!!