the purpose of the syrup for the fountain drinks is supposed to be cheap...
fast foods tend to change that.
I have found however that some convenience stores are the best place for fountain drinks... they charge differently for refills, and will let you "refill" just about any container lol
I bought a huge 100 ounce cup with a lid there for $5.99 about a year ago, and they let me refill it with anything (soda, slushie, tea, flavors) for 79 cents lol... works out cheaper than buying a 2 liter :D