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.
