I don't have a "formula" but I can suggest some things to consider:
(1) Cost of materials
(2) Cost of transportation (going to and from the market for the raw materials - and to and from the event to deliver the finished product)
(3) Labor - prep, delivery, service, and cleanup
(4) Misc - soap and water to wash up the kitchen, gas, water, electricity to cook, etc.
(5) Useage .... if you are providing the equipment (like steam table trays for a serving line) you need to recover a portion to pay for the useage of the equipment
(6) Profit - I assume you're not going to do this "for cost"
In short - what's it actually going to cost you go do everything ... and how much profit above that do you want to make?