When considering barbecue catering bids, ask the caterer to provide in writing exactly hoe many pounds of meat and fixings, and pints, quarts, or gallons of sauce they're offering for that amount. It's quite likely that the cheapest bid isn't the best, in terms of provision amounts.
Before you hire a barbecue catering service, ask whether the provide cleanup both during and after the event; this will probably affect the price. Also, ask them what they plan to do with leftovers; cheaper bidders may intend to keep them.