A couple of additional thoughts to further complicate price comparisons.
A. One probably needs to consider not only what colony the rifle was purchased in (because of the unequal exchange rates that others have mentioned) but, at times, where in the colony. Rifles at Ft. Pitt may have cost more, for instance, than rifles purchased in Lancaster or Philadelphia at the same time.
B. The cost of rifles at Christian's Spring, a Moravian community, may be relatively low because of Moravian business practices or ethics. Kate Carte Engel's Religion and Profit (2009) shows how extensively Moravian church leaders policed pricing policies, insisting that the Bethlehem store, for instance, "aid its neighbors by selling goods at reasonable prices, and not at the highest price storekeepers might possibly demand" (p. 119). In Nazareth, church disciplinary boards routinely dealt with complaints about unreasonably high prices and rebuked shopkeepers for them. So the information about rifles that we can glean from records relating to Christian's Spring may reflect a slightly lower cost than what the "free" market would bear ...
Scott