This paper describes the commonly used 95-percentile billing method which is often used by ISPs to bill other ISPs. The 95-percentile billing method is as follows:
Set a billing rate $y per Mbps (Megabit per second).
Take a month of traffic counts for the entity you wish to bill.
Split the traffic into equal sized time periods of length (often 5 minutes).
Calculate the mean rate in Mbps for each time period.