Once the current “40 litre” (about 400 grams of CO2 from memory) bottle runs out, I’ll be installing the “big bottle conversion” kit - along with the 6kg CO2 bottle - after about 2 x “40 litre” CO2 bottles, I’ll be ahead.
Naturally you have to add in initial cost of the conversion kit and the “outright” purchase of the first CO2 bottle - but if you go through soda at a rate of knots, it’s still not going to be more than 2 years and you are ahead. Along with not having to go back and swapping a smaller 40 litre CO2 bottle regularly.