OK! I'll run my numbers, didn't think about it much when I bought it as it was my pre-mustachian days.
I got my soda stream package with a coupon for $120. The package included the soda stream, 2 bottles, samples, and 1 filled canister. I bought my 5 favorite flavors, those bottles are only like $5. Now I did get the adapter for $60 and in 1 year refilled it only once (at $3 a refill).
120 + 5x5 + 60 + 3 = $208
The flavor bottles I think are estimated to be about 50 cans ( I only fill up the top of the flavors half way and not to the line b/c I think its way to much). So the above is supposedly then 500 cans.
A 12 pack of coke is around $4. I was getting one of those a week... so for one year: $192 or 576 cans.
So the first year I had the soda stream yes...buying the cans would still not have me at even. But then next year:
1 refill, 5 more bottles
3 + 25 = $28 for around 500 "cans"
where as the 12 pack still gives the same 192 for 576 cans
After 2 years:
1152 cans at $384 = $0.33 / can
1000 cans at $236 = $0.236 / can
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More reasonable if I was buying a 24 pack every other week
24 pack of coke ($6) every other week = $144 / yr
after 2 years $288 / 1152 cans = $0.25 / can
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Now this was my pre-mustachian days so silly me didn't think to get the costco card and try to buy a palette or something like that. Those numbers the soda stream would take longer to catch up I, but I would also have to have the room for a palette of coke, grape drink, sprite, dr pepper, and diet grapefruit drink.
What the soda stream mostly did for me was stop my bad purchasing of pop and also I started drinking less of it as well. If I wanted to make a new bottle of pop I would have to actively do it, whereas sometimes bored me just likes to open the fridge and grab the easiest thing to eat / drink. So drinking pop was easier than even getting myself a glass of water (laaazzzyyy). I also use the soda stream with juices and crystal light drinks, it is pretty versatile. Great for making mixers and barely takes up any space and I can have my favorite 5 flavors.
Each side has its pros and cons but until my soda stream breaks it is saving me money as of right now, 2 years after purchase. I'd be really interested to see the numbers on all the home made rigs.