As has been stated, Disney snack prices are similar to sporting events and other theme parks. If you buy ice cream at an ice cream parlor at home, the price is similar. Popcorn in the theatre and a water, similar. For the cost I am spending on a disney vacation, snacks are just a small percentage of the total cost.
So, I will buy the bottled water, and I will not refill it at a drinking fountain, I would rather purchase a second bottle, because the water coming out of those drinking fountains tastes awful!