How are celebrity fees worked out?

Five things set a fee: the artist, the work, how many days, what usage you want afterwards and which territory. Usage is the part most briefs forget — a day's shoot is priced differently from a day's shoot you then run as an advertisement for a year. There is no price list, because a published rate would be wrong for nearly every brief that arrives. What a celebrity endorsement costs takes each of the five in turn.