I started collecting GRRW guns about 6 years ago. Over this period, I've seen a modest increase in interest and prices for these old guns. That said, I tend to agree with Oldarcher and JoeG on the limited potential for significant appreciation in their value.
I'm collecting them for nostalgia reasons and not as an investment. I'm having as much fun learning about the old guns and the people that made them as I am in the pleasure of owning them. Some I shoot and some I don't. Some, the bores are so pitted they're not very accurate and are a pain to clean. A couple are in pristine condition, and I'll keep them that way. Most are in shootable condition, but I still haven't got around to shooting all of them.
One last thought on collecting as an investment, I notice the auction houses are doing a booming business as the leading edge of the Baby Boomers are going to the happy hunting grounds and significant collections are going to auctions as part of estate sales. I wonder though, who will be attending those gun auctions when only the tail end of the Baby Boomers are left. I could be wrong, but it is hard to imagine the folks my son's age putting their mobile devices aside and getting interested in any historical guns whether they be muzzleloaders or cartridge guns, original or reproduction.
Phil Meek