I think one way to estimate that is to see how much media you can possibly consume per month. Obviously you can only watch one movie at a time, and though it is possible to do more, you generally listen to only one song at a time. You probably don't listen to songs while watching a movie. So judging by the number of waking hours minus the hours sleeping and at work (maybe not for music), you should be able to guesstimate the number of "media hours" per month and use that to guess at a flat fee.