Our biggest cost is trophies. Because I have too much on my plate, I always forget to get a sponsor. I have had students in the past willing to take this on and have secured trophy sponsors. A colleague of mine drafted a sponsorship letter that she sends to every organization and department head in her school, as well as known community members. She breaks it down into manageable donations and always gets her trophies sponsored. For example, LD trophies are sponsored by the mock trial team and prose trophies are sponsored by NHS.
Food — Decide on your lunch options and sell tickets that morning. Estimating an amount of pizza to order and hoping for the best generally leaves you with extra. Only order as much as you need to cover the tickets. Some tournaments have utilized food trucks, which eliminates the need to purchase food on your own. I also assign team members to bring in concession items (cans of soda, bags of chips, candy, etc). This makes it pure profit. They also are responsible for providing hospitality room items as well. I create a sign-up genius with food needs.