If cell networks communicated cost-per-gigabyte, then a dual-SIM phone could automatically switch to the cheaper data. This would cause a crash in bandwidth prices, which is why the phone companies would have to be forced into it, probably by the FCC.
After thinking about it for more than three seconds, there's actually two figures of merit: cost-per-gigabyte and width of the desired network connection in bits/s. A gigabyte transfered at a kilobit per second is a lot cheaper than one sent at a megabit/s. Sending a text message from a phone is cheaper than a voice call, which is cheaper than watching a 720p video. So you'd have to have some kind of sliding scale, adjusted by how many other clients there are in the cell, and how much bandwidth they're using...
This seems all very obvious. Has this scheme been described in an RFC from the 80s that I've just never heard of?