It is more complicated than that. Customer convenience must be considered. For instance, I and many others like me will not buy data transfer from someone when figuring the cost requires me to account for the time of day and phase of the moon.
Ever had a cell phone? During the day counts against an allotment, after a certain time at night, calls are free. For a home internet connection it would probably be reversed, as peak usage is right after work lets out.
Some power companies also do the same thing.