What you should do is figure out how much it is going to cost you per pound in finished product (meat, fuel, spices, rub ingredients, sauce ingredients, materials, foil, paper towels, ice, etc...). Once you tally all of that up, you know that you will have charge at least that to break even. Then you determine how much profit you want to make.
If it were me, I would charge per slab rather than per pound. I find that easier to manage, but that's my opinion.