Can Usage-based Broadband Billing Be Done Fairly?

If metered broadband billing is inevitable, what would be a fair construct? Or is it even possible to be fair? Arguably the fairest approach would be one in which the entire bill is variable and in which unit (per-byte) cost declines as usage increases. Fair in that all users pay relative to the quantity of resources they consume but, like any good business relationship, heavier users enjoy volume discounts. This approach might seem too radical for all involved, however; even consumers who save money may look askance. So assuming the above problems could be ameliorated, and further assuming that the "cap-and-meter" approach is the one that prevails, what exactly is a fair cap? 30 GB at least and probably more like 65 GB. (12/27)