The idea gives me slightly perverse glee.
My first thought was "okay, switch to opus for audio to cut bandwidth without sacrificing quality, switch to smaller, lower-bandwidth videos and stop using video for everything, better still, avoid "cloud-only" services and switch to mostly legally-downloadable entertainment and download-and-store-locally instead of wastefully streaming everything every time you want to watch/listen, make more of an effort to reduce media sizes (and stop sending via email, which bloats the payload media by ~1/3 more due to the encoding method), and push back harder against 'involuntary' media downloads (i.e. bloated ads) and "autoplay". Autoplay is a tool of the devil.
Sounds like the incentives that a per-GB tax would impose might be a good thing. ($0.62USD/GB seems like a lot in the US where we already pay a fortune for decent internet compared to much of the rest of the developed world, though.)