I personally price my jobs out on a per meat/side per person based on whether it is a drop off or on site. For example: Pulled Pork = $7/pp, Mac n cheese = $3/pp (not actual prices). For 70 people, I’d charge $700. Charge accordingly dependent upon your area. I’m sure there are some veterans here who may price differently. I just do it this way for transparency and simplicity and it has worked well.
Hope this helps.