Your math is still off. I get closer to 40 liters out of one CO2 cartridge (and that's a low estimate) and 12 liters out of each $5 bottle of syrup.
That means that for 12 liters of soda, I'm paying $0.38/liter for CO2 and $0.42/liter for syrup. So, we're looking at about $0.80/liter or $1.60 for a 2-liter. That's right about there for name-brand soda when it's on sale. At 33.8 oz to a liter, we're talking $0.02/oz for soda. That's $0.24 per 12 oz can. The cheapest I've seen a 12-pack in recent years has been $2.75. That's almost exactly the same price as the same amount of soda from a Soda Stream.
The difference is that I don't have a bunch of trash left over.
Oh, and if I add CRV into the equation, the Soda Stream comes out cheaper.