My first thought is working it as a "cost plus" deal. Give them an estimate and get half up front. Then show them your receipts at the end of the event and collect the rest of the cost and your pus factor. You said you are doing this job as a volunteer so it would seem the fairest way to do it for you and for them.
If not that, giving them a price before you have sat down with their coordinator and gotten all the details isn't wise. Get all the details. Have the EC walk you through the event from start to finish. Get an idea of what she expects.
Make a list in outline format covering all the details, highlighting your responsibilities. Details are important numbers of people, food, condiments, equipment, utensils, table ware, linen, paper even down to things like foil and plastic wrap. From there you make sub-lists. More of an ingredient list or shopping list, how much/many of what.
Then start plugging in prices. Account for 5%-10% extra for unexpected guest and then add a "fudge factor" percentage, for those things you may not be able to control or maybe 'forgot'.
Only then should you go back to the EC and talk price.
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