The price per GB aside from obvious (earns more money) have other reason; it's just so much easier to meter when you have hundreds different services. You just

The price per GB aside from obvious (earns more money) have other reason; it's just so much easier to meter when you have hundreds different services. You just increase the counter. You don't have to worry when service sent data. How much BW the service actually costs is hard to calculate because you are essentially buying peak capacity, so only thing that matters is that hour in a day where there is peak traffic and which services use that up. Sure there is a bit on power used flipping actual bits in the network devices but that's tiny fraction of that.