How much to charge for this cake?

For 100 servings and how much should I charge extra for fondant and gumpaste details? I like to charge by slice, and I usually charge $1.25 a slice for just regular buttercream and not including fondant. Im not sure if thats too cheap. The person is going to come talk to me about what she wants in a week or two and all she said was its for a sweet sixteen party with 100 people. I want to be prepared to know exactly what to charge for, first of all the base price (how much it would be for 100 servings), then the details like fondant and gumpaste, which she will tell me whether or not she wants. It will probably be a tiered cake (2 or 3 most likely). How much would you charge? And those of you who had a cake for 100 servings, how much did you pay for it and what were the details? Thank you!