I then discovered that Coca Cola sells most of their popular flavorings in a Bag-in-Box format, presumably for retail establishments with soft drink dispensers. I then discovered that both Sam's Club and Costco sell these bag-in-box products. Unfortunately, the most popular drinks (Coke, Diet Coke, Sprite, Mr. Pibb, and Dr. Pepper) only come in 5-gallon quantities, although some less popular flavors (Fanta Orange, Barq's Root Beer, Power Ade, Coke Zero, Minute Maid Lemonade, Nestea , and Cherry Coke) come in 2.5 Gallon sizes. The prices, respectively, are around $ 65 and $ 33. The products are also found online for about twice the Sam's and Costco prices. You can see the flavors offered by going to the Costco web site and search for "Coke Bag in Box"
One problem is that these products have a special fitting for attaching to the retail soda machines. However, on Ebay, I found the following product:
I used it to fill several 1 liter bottles with syrup which I keep in the refrigerator. I don't think the bag-in-box needs to be refrigerated. It is not purchased that way and
I have seen photos of restaurant installation with several bags-in-box in an open air rack and hooked to drink dispensers through hoses.
The bag in box drink mixes are mixed in a 5 to 1 ratio with soda water, so a 5 gallon bag in box produces about 30 gallons of soft drink. This is not too much of a problem for me since I mostly drink lemon-lime soda (Sprite). This works out to about 50 cents per liter and nothing else I found online even comes close.
Moreover, these drinks are the exact equivalent of those that you get from the fountain machine at, say, McDonalds. In fact, looking at McDonald's soda machine, the available flavors appear to be the very ones available as a bag-in-box. No surprise there.
I am now very happy with the convenience and the net drink price of my Soda Stream. I just wish all the Coke flavors came in the 2.5 gallon size.