Al Frasca (trapdoorcollector.com) is the gold standard for pricing. Right now, for example, he has an 1884 infantry rifle made in 1885 that he describes as "minty" for $1595 + S&H. You can sometimes catch a more-common rifle for $1100-ish. Complete, as issue, no worries. Mr. Frasca wrote the book(s) on 45-70 trapdoors, and is a well-known collector.
He also has parts, sometimes, and there are some others who list parts in his "classifieds". Figure you're probably looking at a minimum of $300 shipped for a correct (correct variation for your action and barrel) replacement stock. The only real source of parts is from rifles that have been parted out, now or in the past.
Stepping down to Gunbroker (a "step down" only in the sense that it is a step away from almost-exclusively collector-grade arms), for the past few months shooter-grade as-issue rifles have been going in the $700-1100 range. Cut-downs for less, rifles with particularly good finish or bore (or salesmen) for perhaps a bit more. This is getting into the range of Mr. Frasca's goods . . . .
I bought mine via Gunbroker a couple months ago. Total with shipping was around $750 for mine. Just a shooter-grade rifle: stock refinished, metal treated to a rattle-can black finish, perfect bore and the sights I wanted. I also watched for several months, and knew exactly what I was looking for ("perfect bore" and "sights"). A can of airplane stripper took care of the paint. I lucked out, no parts were missing or broken. (Over the years, the only parts I can recall breaking or acquired broken were an extractor, a firing pin, sears and notches in tumblers, and a bent barrel (that last on an 1868 50-70, more's the pity). Missing parts are another story!
