I recieved a request for a Number 2 birthday cake. I have the newish Wilton set where you can make numbers and letters. I haven't used it yet but have a game plan how to make this cake. Problem is giving her a quote before she agrees. She wants it to be marble sponge with Bavarian creme filling and vanilla buttercream frosting. And to serve 12-14 people. I've done research in the past and read posts on here about charging $3 per slice, but isn't it for a.set size? Will she be able to get 14 slices out of this cake? How much should i charge her?

Any advice is appreciated! 

Does the box the pan came in tell you the volume of batter the each section holds?  If so, you could compare that to the volume of batter needed for a regularly shaped cake (say 8 inch round) and compare that to how much that regular shape would serve.

Not knowing the exact pan size that  you plan to sue it is hard to help you.   As the other poster said, compare the volume of batter your pan holds to other pans to see if it will yield the # of servings you need and then price according to what you would get for that size.  Most pans that use the amount of batter that one box of cake mix gives you will certainly serve 12-14.  Price accordingly.