It doesn't work out any cheaper than buying it , but it does cut down on the plastic, which is mainly why I bought one. Also, I make a bottle at a time and it stays fizzy whereas it would take me longer to drink 2l and it would start to go flat.
I bought the classic hydration kit f0r £80, so it came with 2 large bottles, 2 small bottles, and 2 flavours and 1 gas cylinder.
I just mix it with squash, or elderflower cordial. I have never bought their flavours as they are far too expensive, although it did come with a lemonade one which was nice.
I have never bought one that uses power, but I presume that you don't need to buy the gas cylinders so that would save money.
The gas is the biggest cost for me, I order 2 at a time to save postage, but it means spending nearly £50 in one go , although I did recently discover that the local sainsburys will exchange them, so that would only be £12.99 as long as I return one at the same time.
If you use it a lot though or more than 1 of you, there is a monthly gas plan, which makes it a lot cheaper.