Here's an excellent piece on the costs of delivering internet bandwidth.
However, my math comes up with some different figures.
Three hours of content at 800Kbps costs, wholesale, around 10c (the networks would be able to average this down to under 5c using their own networks, I would guess) - so the scenario in this piece - someone watching for eight hours a day for a month would cost around $8. Considering that most people already pay $19.99 or more for their broadband, double charging this does seem somewhat excessive.
This would also result in discouraging the rollout of multicast networks by ISPs, which would only use a fraction of the bandwidth, but has further capital investment costs which could not be covered by the current proposals for 'premium charging'.
However, the broadband market is so competitive I have little doubt that the notion of 'premium charging' just won't stick in the marketplace, whatever the lawmakers in Congress do.