If the intent is to make soda and not just carbonated water, I'd say not only is it not worth it, it's a super bad idea, even if it costs $1.00. Better to just kick the soda drinking habit, period. To me it's like asking if a machine to roll your own cigarettes is "worth it." Yeah I know people are going to do what they're going to do, but I'm still going to be the downer who rains on the soda-drinking parade.
Sodastream, and it's consumables, as sold, is not an MMM 'value' unless you drink a LOT of soda.
There are multiple ways to do it cheaper.
The cheapest price per glass is from buying (and refilling) a HUGE CO2 tank. But that has consequences
I am in the process of assembling a home-built water carbonation system.
I got paintball CO2 tanks off craigslist. I will fill them myself by buying dry ice and using a digital scale.
I got the valve for the tanks from eBay.
I'm doing this because diet coke is my only vice, and only source of caffeine.
Making my own will allow me to control which and amount of; sweetener, flavorings, caffeine and no coloring(s), etc.
I concede the well intent of your post, but I fail to see any constructive content.