Pricing is one thing---cooking for a party is another
Let's address the cooking:
Each meat cooked at an event needs enough of that meat to feed each person at that event. Shortage of brisket can not be overcome with sausages and turkey breast. Cook enough brisket for 25 people----if you cook sausage, also cook enough for 25 people. If your coworker wants your brisket--that means he likes your brisket, and he has talked about your brisket, and his guests will want that brisket also. When the brisket runs short-----all upset people that didn't get brisket will blame who??-----it is the COOKS fault. If you want to do more partys later---this is not the way to start.
Pricing:
If you are a caterer, with insurance, proper certificates, licenses, proper equipment for holding heat while serving, etc----then charge like a caterer. If you are a coworker cooking for a friend do not charge a "catering" fee--this can lead to other problems. I would tell your coworker the cost of all groceries, spices, BBQ pit fuel, vehicle fuel etc. Asking for reembursement for supplies bought causes no problems. Then look at your motives for doing this work---are you wanting to do it for financial gain, are you wanting to do it to start a catering career? Are you doing it as a "friend"? Are you doing it to display your "Name" as a cook?--Did you have to take time off of work, or anything else that would "cost" you money to cook at this event? I would simply state to the coworker---preparing, cooking, and cleaning up for this event will use up about X amount of hours and let them decide what to pay. If they have asked "How Much" then they have showed "intent to pay" and expect to pay-----often times folks will hand you more money than if they were quoted a price. Especially after they see the work involved