Since infrastructure is the same initial cost to everyone; I would guess that electricity is the main cost. Driving continuous max bandwidth should require more energy and thus cost more.

I say this without knowing too much about it; but I do know the cable line is separate from the power line... so the cable company needs to drive the bits through the coaxial with their own electricity, correct?

A long, sustain, high bitrate seems like that would use more, no?

Just speculating...