Lets assume Comcast honestly thinks that Netflix owes them money for the bandwidth. I know it's highly generous, but it's a fair starting point.

How does it costs Comcast less money to peer with Netflix than to get it from Cogent? Does Comcast pay Cogent too?

Of course the whole confrontation was manufactured, I don't think Netflix was ever arguing that Comcast needed to boost its entire buildout just to support their video streams, and by the way they shouldn't pay a cent. Comcast doesn't have to wait till its pipe are full before controlling traffic flows.