But you don't have the other variable to plug into the formula, the amount of profit, as you do with the other costs.
With a light bill, you can't plug in the KWH until the bill arrives. Nor can you always know what rate is being used, as the rates vary for a number of reasons.
With a copier bill, you can't plug in the # of copies until the bill is arrives either.
So how is it different besides being it being something you want to think isn't paid with sales revenues?