I love my Sodastream soda maker (you can read a review I wrote a couple of years ago here), but hate having to spend $20 for a couple of dollars worth of CO2 gas everytime it runs dry. The Sodastream has a proprietary connector design that locks you into using their custom tanks which allows Sodastream to charge a monopoly price for a commodity product.
That's their business model, and more power to them, but if you're not keen on being price-gouged on gas, consider the SodaMod, a small brass adapter that lets you use cheap paintball CO2 tanks in your Sodastream. A paintball tank will set you back around $20 and will cost just a few dollars to refill with CO2 at your local sporting goods store or paintball field, saving you ~$17 per fill up.
The SodaMod is $60 but will quickly pay for itself with those kind of savings.
For the more adventurous, here's a video about how to refill the Sodastream tank yourself using dry ice. This way seems a bit more dangerous as you have to make sure you don't put too much dry ice. The Sodastream tanks have a safety valve that would presumably blow if it did have too much gas, but even still, better to play it safe there. Just don't be stupid.
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