There are a few that fall out of the woodwork every year. you might acquire a local gun or squirrel rifle for $10,000 or so. A full blown half stock plains rifle might fetch $40,000 and up. One with known provenance and ties to a known mountain man might command $75,000. . My wife keeps pushing for me to get rid of mine, but I am not tired of enjoying them.
Like any antique, the actual price is what a willing seller and a willing buyer agree too