I too have been working on a BP for catering/comps for quite some time. There are some costs you are missing. My comments that follow are what I've discovered here (Boulder County Colorado) over the last year.
OK, even though you may have commissary privaleges somewhere doesn't remove you the sink requirement for hand washing. If you think that through, you need a water source, a heating source and a gray-water tank. If you don't have commissary privaleges, start think addition sinks (dishwashing, rinsing, sterilizing, and mop). Claim that you won't be washing things? How about cutting boards, knives, pots for heating beans etc. How about the grates in the cooker? It gets complicated quick.
If you're serving beans as a side, even just canned beans, you have to have a way to rapidly heat them. Putting them in the pit won't cut it as far as my HD is concerned. So now you need a propane burner/stand. Even the hot dog carts here have to have a hand washing sink.
Ice is an approved way to keep things cold that need to be kept cold, but there is a requirement that all food storage has to be so many inches off the ground. Now you got figure a way to keep the coolers off the ground. Not a biggy but something else you got to lug around. And what to do with the melted ice? FYI, letting it sink into the dirt isn't approved.
To sell at comps here, you have to a food service license. Now I'm beginning to understand why there are so many non-vending teams at these things.
And there is the NSF thing. Is your pit NSF certified? The local HD here said they would evaluate the pit when I submit my application since I haven't bought it yet. Looking at brochures won't work and they won't commit to an approval until they can look at and touch it. What if what I purchase won't cut it? Out several thousands of dollars I guess. Hence, my interest in the FECs and FatBoyz. That NSF certification is gold to me.
Other hidden costs, or at least questions you should have an answer to is disposing of grease, gray water, and trash (eg cryovac wrappers, paper plates). I think insurance has already been mentioned.
I've decided to go totally above board, even though most of the folks that I know that "cater" claim the Personal Chef thing. And then they are not fitting the requirement of only cooking at the clients house using only the clients equipment (ovens, stoves, knives etc) as they all cook at home and them haul the prepare food to where they are catering.
Interestingly, no in my neck of the woods is catering BBQ save a few resteraunts that deliver. No one that is "legal" is cooking on-site.
All that said, I've looked at the prevailing prices that my competitors are charging. I've looked at my projected costs (upfront, ongoing, and hidden) and know that I can make a profit. Will I get rich overnight? No, but I think I can make a comfortable living doing this in a few years.
Anyway, I hope some of what I wrote is helpful to you.