Take your costs and multiply x 3 for a fundraiser, non-profit kind of thing.
Say you're serving a 4 oz sandwich, quarter pound that is, you lose around 40-50% of raw product, if you're paying around 1.50/lb for shoulder meat (might be cheaper down south), then you're spending about 75 cents per sandwich in meat. Bun, 10/15 cents, sauce, 10/15 cents, rub, charcoal, etc. $4 sounds about right. Just check your numbers.
Make sure you're either set up for catering legally or be sure your a temporary employee of the Rec. Dept so you don't get sued if someone claimed to get sick of your food. Be careful and good luck!