Having done the cooking for a LARP weekend for 60 people…
You will be catering for several groups of people. For example I had to cook for the non-fussy eaters, for vegetarians, vegans, for a gluten intolerant person and someone that had a sever allergy to rosemary. So for the first night I had to have 4 different dinners cooking and had to alter the recipes that used rosemary as that was totally removed from the venue.
Some basic rules
Vegetarians can eat Vegan food. Some will also eat cheese etc. If in doubt cook vegan for all of them. Check the labels carefully. While vegans are used to picking around meals to see they can eat, if you can make sure that you cook one or two things just for them.
Oven bags allow you to cook Vegan food at the same time as some animal products.
What you cook also very much depends upon what period the LARP is set in. Because I was doing a medieval LARP I tried to have no potatoes or other non-european fare. Thankfully they had rice in Europe at that time.
Breakfasts were
Porridge
Cereal
Toast - two different types of breads
Butter, margarine, jam, honey
Baked beans for those who wanted a more robust start to the day. Plus a little bacon.
Lunch one was a large cold meat platter with bread and vegetarian options
lunch two was soup and left overs
Dinner one was a very meaty stew for the meat eaters and a classic pottage for the vegans
Dinner two was
Course one - Garlic and Herb bread made with home made breads
Course Two - Soups
Course Three - A Salmon fish ball dish
Course Four - First dessert of very rich ice cream with a boysenberry puree
Course Five - A full beef roast course with roasted veges
Course Six - Dessert two: A fresh fruit salad with Chantily cream. It was meant to have a custard with that but that got missed.
Drinks were coffee, tea and lemon cordials
plus I had brewed three different types of beers for consumption with the second dinner.
Total cost for the weekend excluding alcohol was $14/ head. Unfortunately most of this was not cooked from recipes but rather from instinct and the help of an ex-professional chef, which saved my bacon at least once.
Careful planning and looking what is in season to cook with will keep the cost down, but I doubt I could do it for less than $16/head today.
I hope that is of some help. If you have other questions feel free to send me a pm.