Even the cup costs pennies. You’re paying for the movie theater. As I understand it, the ticket price mostly goes to the studio and the distributor; concessions pay for the facilities and operating costs (and profit for the theater owners).
I always ask for tap water. Some places are great and give me a soda cup with ice and everything. Others are resentful and give me one of those tiny paper cones that holds about two sips of water, gets soggy in 30 seconds, and can’t be set down on a table.
These days I mostly go to the fancy theater where I can have a beer. “And I don’t mean just like in no paper cup, I’m talking about a glass of beer.” For about the same price as a soda at the multiplex…