They should simply do it like this:
* If you use X GB of data a month (whatever is the average for heavy smartphone users), it's included in the everything plan (and if you go over without tethering, it's also included)
* If you tether and go over that avg. use, you can purchase a "hotspot" extension for $29.99
That works for everyone. Because really, if Bob doesn't tether and uses 2 GB of data and Sam does tether and uses 1.5GB of data, why should Sam pay more than Bob?
* If you use X GB of data a month (whatever is the average for heavy smartphone users), it's included in the everything plan (and if you go over without tethering, it's also included)
* If you tether and go over that avg. use, you can purchase a "hotspot" extension for $29.99
That works for everyone. Because really, if Bob doesn't tether and uses 2 GB of data and Sam does tether and uses 1.5GB of data, why should Sam pay more than Bob?