For the last few months I've been experimenting with ways to use the SodaStream for flavored drinks and keeping track of how much of everything we use so I can calculate costs. Here's what I've found!
New SodaStream C02 Canister: $30
Liters of Carbonated Water Per Canister: 62
48¢ per Carbonated Liter
Near the end of our last C02 canister we realized we didn't need to use as much C02 as we had been to get a decent level of fizz, so we started using about 1/3 less C02 per carbonated liter. I suspect future canisters could carbonate more like 90 liters, which would make it 33¢ per carbonated liter - MUCH better!
You can buy 2 liters of seltzer water in the store for around 80¢. So even without using any flavors the SodaStream is more about the convenience and not wasting all the plastic from individual seltzer water bottles.
I could see some people saving a little money over the long-term IF they had been buying a lot of seltzer water from the grocery store. (As much as I love carbonated water, I hadn't been buying it from the store since we had perfectly good water coming from the faucet.)
But what about once you add the flavoring? Soda is more expensive than seltzer water....
We've tried a few flavor options:
Monin Flavored Syrups: $9.95 per 750 ml bottle, 2 tablespoons per liter: 39¢ per liter
SodaStream Flavors: $4.99 per bottle, makes 12 liters: 42¢ per liter
Homemade Flavor: 18¢ per cup of sugar, 99¢ per grapefruit, makes 8 liters: 14¢ per liter
The cost of the flavor is in addition to the cost of the C02:
Monin + C02: 48¢ + 39¢ = 87¢ per liter
SodaStream Flavor + C02: 48¢ + 42¢ = 90¢ per liter
Homemade Flavor + C02: 48¢ + 14¢ = 62¢ per liter
I can regularly get 2 liters of soda for 99¢, which is 49.5¢ per liter. NONE of these options is currently that cheap.
If using less carbonation does get us around the 33¢ per carbonated liter mark AND using the homemade flavorings that would be 47¢ per carbonated liter, which is hovering around the same price as store-bought soda.
Of course, the homemade soda is much better for you then store-bought soda, since I use real sugar and natural flavors.
Conclusion: The SodaStream is not considerably cheaper for seltzer water, and is definitely more expensive than buying 2 liters of soda from the store. But on the other hand, it is much more convenient, and uses much less packaging.