All the fast food restaurants and quick shops (the ones that sell gas) all charge the same for pop or for water and/or ice.
I've asked why, and was told they calculate for ordering quantity based on the number of cups sold. Don't know if that's true or not, but I've been told that at Wendy's and Arby's recently, when I wanted plain water. And I've been told the same, when I've bought pop in a bottle and asked for a cup of ice in a quick shop.
In the summertime, lots of the quick shops will sell the giant cups of pop for 99cents. In the winter, they do it with coffee. Pilot started it, and most of the rest followed it, cause they were losing business to Pilot.