In UK we would generally charge by the ingredients, the complexity as well as how much the cake decorator would want to charge per hour for their labour. (Not forgetting the overheads to run the business in the first place. ) We do not so much charge per slice but for the whole creation. Although of course we would reiterate how many slices were contained within the cake design chosen.
I charge per serving PRIOR to the cake being shaped to ensure there are enough servings after. So if they need a cake that serves 50 and the cake shape will require a lot of cutting (thus losing cake in the process) I bake a base shape (square or round) that is abt 15-20 servings more.
The price adjustment helps cover the "loss" of cake being cut away and the extra time on shaping.