If you're willing to get your hands a little dirty, there's an alternate option that doesn't involve buying into a razor blade business model.<p>At a welding supply shop, they'll sell you a 5lb tank of CO2 (around $60 initially, then $15-20 for each refill thereafter). If you then go to a homebrew supply store, you can buy a CO2 regulator (~$70, depending on quality), the appropriate tubing and a ball-lock adapter (< $10), and a carbonator cap ($15, something like <a href="http://morebeer.com/view_product/18250/" rel="nofollow">http://morebeer.com/view_product/18250/</a>). That setup'll let you take any plastic soda bottle (1-liter, 2-liter, whatever) and carbonate it directly. I have a CO2 tank for kegging my home-brewed beer and soda, and I frequently use my carbonator cap to make small test batches of soda before mixing up a full five gallon batch in a keg.<p>A 5lb tank of CO2 will last for 15-20 5-gallon kegs of beer. There's no easy apples-to-apples comparison (soda is more highly carbonated than beer, and I'm not sure of the effect on efficiency from carbonating such small batches), but any way you work it paying $15-20 for a refill is a much better deal than paying for SodaStream cartridges.