^^^ absolutely on the purchasing. Drive your costs down. Watch every penny. For that matter, see if you can purchase your meat through/with a local restauranteur who gets meat directly from a distributer. I do this for our whole pork shoulders that you cant purchase elsewhere. I end up paying usually around $1.10/lb (or less) for Smithfield. Cut out that middle man or two; it'll help tremendously.
My wife keeps wanting me to vend at a few local events that truly could use good barbecue. Frankly, at $5/sandwich/plate we'd work our butts off and *possibly* make a few hundred profit; possibly... And the effort that would go in to this, for a few hundred bucks? No thanks. What Jacked UP BBQ said is dead on. Hobby and a few bucks, fine. Otherwise focus on the larger guaranteed things. That way you know ahead of time whether and how much you'll profit and you can decided whether it's worth the effort.
Hance - Lake Dogs Cooking Team - MiM/MBN/GBA CBJ and comp cook
Lake Sinclair, GA (strategically about an hour from everywhere)