The school has a budget for camp. Most likely your co-teacher is 'in charge' of the camp and will have a budget for the whole thing. If he/she asks you for a plan for something then make sure you ask her about the budget for that activity. If there is no budget then make an activity that requires no budget.
If you are doing a cooking activity then make a spreadsheet with the 'quantities per recipe' and the 'recipes per bag/can/box/packet' of ingredients. This will help you plan how much to buy for all the students in the camp. In our town there is a supermarket which specialises in restaurant supply. It means you can buy giant bags and cans of stuff more cheaply than at a regular supermarket. If you find such a place then you might be surprised at how cheap the total cost works out. Actually, some supermarkets have a tiny section full of 'catering sized' packs so it's worth looking around the whole store. The best deal I found was for spaghetti. It's something like W2500 in the supermarket and W800 in the shop I found. This was a significant cost saving. I also found I could buy frozen minced beef cheaply, and frozen minced pork (can you guess the recipe?). By mixing minced pork and minced beef I could make up the quantity required, and keep the cost down whilst preserving the taste.