Make yourself a budget for fund raisers each year, almost every business I know has a budget for this sort of thing. If you are involved with the charity then do what you feel is a fair deal for them and you but if you are not involved and they are just looking for a guy to do the cooking then have a set price. It is really hard to do anything for less than $500 considering the time, rub, fuel etc. so that is my base for non-involved charities. Military cooks get me for free.