what you would need to do is figure out what size pans your are going to use, and then you find your number of servings and then you also go by your decorations such as if its fondant its more. because fondant is more costly then buttercream icing. and also difficulity. once you sit down and fiugure out your sizes and servings you can figure it up or around about from there.
. There is a post simlar to yours here. if I can find it there is a link in there that I will be happy to send you.that might be of more help to you.