For the last thirty years at least the rifles have been significantly more expensive than the shotguns. The near exception has been top quality samples in exceptional condition where prices can be fairly close. Even there, rifles tend to bring more. At least two factors are involved. First, the rifles are far less common. Second, I think they involved more expense to produce. Nowadays the best source for finding a good percussion double rifle is probably the major sporting arms auctions. They don't appear often on the open market. As an aside, I'll point out British rifles were often built to shoot a belted ball or another specialized bullet, and the rifles are often separated from their moulds. From experience with two doubles made for belted ball, I can say both shot quite well with round balls. Good luck with your search.
