Fox,
Are you looking for a percussion or a flintlock? Percussion generally costs less. Are you looking for Historically correct or just a " in the spirit of History" rifle, Historically correct generally costs more. Custom hand built rifles can be very expensive. When Inlines became legal to hunt with, many guys sold their percussion "In the Spirit of History" Hawken style rifles that they used for hunting. If something like that would meet your needs, you may want to look at a used percussion Hawken style rifle. Thompson Center made some very well made ones but those still command a higher price. Traditions made some percussion lock rifles with a synthetic stock that shot very well but did not look "traditional". My Wife still hunts with hers and they were very accurate (just did not have a nice wooden stock). Those can be picked up relatively cheap. Be sure to check it out for rust in the barrel regardless of the make. Scratches in the stock can be refinished but a rusted barrel probably should be avoided. Check out some Gun Shops and see what is out there.
Good luck on your black powder quest,
brian