I am not sure how each and every concession contract is outlined but typically, the company that provides the concessions only gets a small percentage of the actual sale. Out of that percentage they pay the staff and the cost of the product.
Back before Jeff Hunt took over the 67’s, it was separated into thirds. One third went to the City for the “rental” of the concession space. One third went to the Rough Riders because they held the food and beverage rights in the entire LAnsdowne PArk. The final third went to whomever the company was at the time that operated the concessions. So if there was a $3 bottle of water, the company running the concessions only got $1. Same goes for beer and all the other stuff. That one third needs to pay for all the variable costs (labour/food) etc.
At that same time, Kingston ran their own concessions out of the old rink and prices were way cheaper. This was because there weren’t three hands in the same pocket. Now with all the new rinks and concession deals. Prices are outrageous because of all the grubby fingers in the same pie.