Proration and special device fees sound like a good idea on paper, but complicates the way we try to do business. We prefer to keep it simple ("$6 per active device") instead of complicated ("$6 per device if you have it active all month and use it every day, or $3 if you only take it out once every few days. Or somewhere in between.")
This complicates matters when you have hybrid devices that don't count as cell phones (Internet of Things devices, routers, etc) but still check in with the towers and use some services.
Time and again we've seen that the simpler solution is always the better one when it comes to explaining how it all works. And then we don't have to open up a support ticket every time someone feels they shouldn't pay the whole $6 when their tablet pinged a cell tower once in the month, for example.
We have experimented with reduced line access in the past. When we partnered with Tinitell, their access fees were only $3. It didn't seem to make much of a difference in those customers' bills, as they tended to have at least one phone with enough usage to more than cover anything the Tinitell watch did.
We can work out special pricing for business accounts, if that's what you're asking. If that's what you're looking for, check https://ting.com/business for more information on submitting your proposal to the business sales team.