"Say 500 MB a month at 3G speeds"
That seems a sensible suggestion - 3G Speeds are fine for most truly mobile users. However, with all the 'content rich' web pages out there 500MB is so easy to pass (Auto-start video is a particular hate of mine).
There is also a lot of merit in the 'at reduced resolution' idea - who in the hell *needs* to watch 1080p or better on a 4.5" screen? What's the point? If you think there is a significant difference, please tell me as I would love to sell you some oxygen-free copper speaker cables.
The interesting thing here is that people using 4G and the allegedly-soon-to-be-useful 5G as a replacement to traditional
hard-line xDSL,DOCSIS or FTTP may have a different POV.
Telcos please note : Live by the (bandwidth/speed) sword, die by the (bandwidth/speed) sword.