fountain soda barely costs anything, this isn't about saving money so much as it's about extracting more out of the customer

About $7 over the course of an entire day of operation. That honestly might be too high, come to think of it

https://en.wikipedia.org/wiki/Free_refill#:~:text=Due%20to%20the%20extremely%20low,refills%20as%20a%20sales%20gimmick.

Due to the extremely low cost of fountain soft drinks (especially the beverage itself, not including the cost of the cup, lid and straw), often offering a profit margin of 80-82%, establishments tend to offer free refills as a sales gimmick.

When I worked at Pizza Hut (early 2000s), for shits and giggles I calculated the actual cost of a glass of Pepsi. Accounting for the syrup bag, the CO2 cylinder and water (the latter being the cheapest since it was filtered tap from the city main) it was like $0.14 a glass.