A slightly different take, if you are not a supporter of the cause, but, feel that if you don't do it, they can't pull it off, then do it for cost plus a simple fee. I would say cost plus $75-$100 for a day of cooking would be fair to me.
About the only time I cook for crowds anymore is for charity, and despite the price I pay (physical pain, exhaustion) I never charge, but, I won't cook if I don't support the charity.
Whip It Off, Chambers!
"perhaps...but then again...maybe not..."