When I was in the networking biz, i was informed that the cost of the cable is insignificant, compared to the cost of installing it.
I would wager, having terminated more than a few RJ45 cables, that the cost of (correctly!) installing connectors on each end, dwarfs the cost of the cable in between.
Oh, and RJ45 connectors are designed for solid copper wire, as almost all telephone wire (in the US at least) is solid (26AWG?) wire. The ones for stranded wire are the "special" ones.