FWIW, you can buy a sodastream unit for ~$70 and then for another $200 to $300 get a conversion kit and a large co2 cylinder and from then on the cost of carbonation works out to less than three cents per liter. Not sure what the syrup cost is since I only use mine for sparkling water.
I just wanted to comment to say that that is a brilliant idea. I only use my Soda Stream for sparking water as well, but I pay about 25 cents per liter for the standard refills.
I have a co2doctor[0,1] kit, a 20lb capacity tank from amazon (bought new - you can get them used but then you don't know where they've been or how clean they are...) and filled it for about $30 at Carbonic Service[2] in Santa Clara.
