Internet usage billing

Over the years we have seen providers using various methods to bill their clients for internet usage. . . . .
Some resort to time based billing whereby whether or not you are doing anything useful online you are billed for time spent online. . . .sounds sensible but more often than not you get excruciatingly slow speeds. . . . .
another system is based on traffic that is commanded through your connection. . . .again it makes perfect sense especially for those who dont like paying for what they are not using.
but this scheme has one drawback. . . in this day and age of multimedia rich sites and resources. . alot of data is downloaded even just to view a simple site. . . .
this is where I have a proposal for service providers on a new billing system.
I suggest they should bill for actual useful work done. . . ie. . if i start a download then it times out due to a network failure I shouldnt have to pay for 90% of a failed download. . . . withou a doubt such a scheme will never be implemented by the mainstream providers but would be an intresting offer if ever implemented. . .