Geez.
PLEASE just charge us all per GB. How much did you use this month? Multiply it times $x and there's your bill.
I know people don't like the idea of their bill changing every month, but why should internet usage be any different than water, gas, or electricity?
Could you imagine if the electricity company charged us all the same (too small) amount and then tried to get air-conditioner manufacturers to pay them more? Insane. Just charge us what the service actually costs. If you need more money then you need to charge us more. This isn't hard.
AT&T got halfway there, but they still have different plans. No. No plans. Just $x per GB.