If only it were that easyI'd suggest by looking at your total expenditures without food, and use that as a base. You need to consider so many aspects when planning a menu and pricing it that there truly is no easy answer. I would suggest you get menus from caterers that you consider to be the competition and review their pricing in order to give yourself a ballpark, but look at seasons, ingredient costs, location of party, rental equipment, labor, prep time, and the other 800 variables before you nail yourself into a price structure. And remember, it's easy enough to change as long as you don't over-promote a priced menu!!
If no one will follow you, you can't be the leader.