It doesn't cost them anything to transfer some amount of data, but it does cost them in the total amount of bandwidth they have left to sell to other customers.

It doesn't cost them anything to transfer some amount of data, but it does cost them in the total amount of bandwidth they have left to sell to other customers. At some level transfer quantity and rate are the same thing. A bit sent for customer A takes up space on the wire that can not be used for customer B, no matter how it's measured. It's also unclear which line would be measured for purposes of Netflix's transfer rate. That's one of the beauties of the cloud... specific line transfer rates are abstracted away into an aggregate quantity of data. If you're selling lines, you charge by rate. If you're selling transfer, you sell by bits.