I bought a sodastream source off Facebook for $40 because my household drinks gin and tonics most nights which has a few issues: all the plastic piling up, 2.25L bottles going flat if we skip a few nights, and having to make extra supermarket trips or buying a bunch of bottles on the weekly shop. So far I've been really disappointed by the economics of it. The carbon dioxide from the standard refills works out at about $0.50 a litre, and the cheapest tonic syrup is at best $8 for the Sodastream branded 500ml, which is $1.50/L on sale, but also in a plastic bottle which is a bit annoying if trying to reduce plastic, all the way up to the Soda Press Organic Tonic syrup at $3.20/L on sale. Oh Bubbles make a syrup that's in the middle price-wise. I've had the Barkers syrup too which I've been making a bit weaker and that's about $1.90/L although I usually make it a bit weaker than their prescribed amount.
So at a minimum it's $2/L, meanwhile at PaknSave I can get the Schweppes standard for about $1.66, or Pams for $1.25.
I know there's adaptors for sale on TradeMe for refilling canisters elsewhere although not sure what that costs but that doesn't really solve the expensive syrup problem.
Some more practical ways to save a few $ other than filling your own.
It also seems to make a difference how much you press the button. I notice my kids hold it down for ever and we seem to rip through the canisters when it doesnt actually need that much and if your a lot more careful it lasts a lot longer.
One way that can work to save some money is with Briscoes. When they have 60% off everything in store type sales - this usually makes the refills a lot cheaper than other places.
Also I often ask for canisters at the counter- dont offer to swap one. Some of the time they give you one and either dont know its a swap or forget to ask - as such I have like 5 of them now, which means a big fill up with the sale comes. I often also see them in hospice stores and the like for very cheap. Its a lot better to have a few of them than just one.