I did forget to add that when doing a carved cake, I charge per slice of the cake BEFORE any was cut away at a lightly reduced rate since in order to carve, it didn't need to be iced smoothly and look perfect.
You have to take a really educated guess at how much you are carving out and how much you are leaving there to determine the servings. You can sort of measure and divide by the size of the servings. A bit o' math for ya!
Of course, a carved cake is so much more work and you end up adding on hrs and hrs of labor. Therefore, they end up being much more than a regular cake for less servings.