The current model for charging for traffic in the Internet is that sites connect via some "point of presence" of a provider, and pay a flat fee per month according to the speed of the line they attach with, whether they use it to capacity or not.
For existing applications traffic and capacity, this model is perfect. Most sites wish to exchange data, which has value that is not increased radically by being delivered immediately. For instance, when I send electronic mail, or transfer a file, the utility to me is in the exchange.
The network provider maximises their profit by admitting all traffic, and simply providing a fair share. As the speed decreases, my utility decreases, so I am prepared to pay less. But the increase in possible income from the additional users outweighs this. The underlying constant cost of adding an additional user to the Internet is so low, that this is always true.
However, there are other kinds of traffic, that this "best effort" model does not suit at all, and we look at those next.