I use one of these large 2.6kg co2 cylinders and an adapter to refill the sodastream cylinders myself at home.
A sodastream refil is $19 for 400g of co2 ($47.50 per Kg of co2)
Filling your own for the first time is $86 for the 2.6kg cylinder and adapter. ($33.08 per Kg).
Then getting the 2.6Kg co2 refilled when it runs out is about $36 ($13.85 per kg)
Not for everyone though as the co2 bottle is very heavy and it’s tricky to learn how to refill the sodastream cylinders. The newer sodastream cylinders have a fancy valve that tries to prevent exactly this sort of thing as I’m sure they’d rather you keep buying their refills.
It’s also potentially fairly dangerous, you need to store the co2 properly (in a well-ventilated area) and you’re dealing with very high pressures (eye protection) and cold temperatures (wear proper gloves).
