With that reasoning people that use it very little should pay very little, while people that use it more pay more. Something similar to the Electric meter on the side of every house. If you are using the equivalent of 14.4kbps of your available bandwidth you only pay for that. If you are using the full 256k or whatever you have, then you pay the full price.
The problem with home users is that there are no SLA (Service Level Agreements) with the Provider.