We did it one year at Universal when we stayed 3 nights. I think they are something like $7.99 to buy then 99c to refill.
I prefer to take bottles of iced water into the parks, I buy a gallon of water for $1.99, refill bottles and freeze each night or buy 24 bottles for $3-4.
Didn't know about taking it to SW & Aquatica.
Wrong parks but we did a test this time how much drink you actually get in a Disney soda. We took some bottles of drink in with us then asked for cups of ice. They gave us regular sized cups filled with ice, we poured in the drink from a 500ml bottle, could only fit in about 125-150ml. Reason when we do buy Disney drinks we always ask for no ice, they are always cold enough without and you get about 3 times the aount for your money.