The selling point is that it's an overpriced "boutique" GADGET for rich peo= ple to waste their money on. It's something fun to do for the first few tim= es, then you get bored with it, then you get sick of it taking up space on = the kitchen counter, then you throw it away or donate it to the local churc= h rummage sale.
It has nothing to do with taste, price, or convenience. The Sodastream flav= or doesn't taste like either major cola brand. The Sodastream machine is st= upidly expensive for what you get. Then, what is more convenient than twist= ing the cap off a 2-liter bottle and pouring?
Frankly, if you go through enough cola to justify a carbonation machine and= formulating your own syrup on a purely FINANCIAL basis, you really need to= revisit how much cola you drink.=20
Keep in mind you're paying RETAIL for small quantities of ingredients not c= ommonly found on the consumer market, while a bottler is purchasing them on= an industrial level at wholesale.
As an academic exercise it might be a worthy challenge to see if it's possi= ble, but you're not going to exact any real monetary gain unless your famil= y goes through thousands of liters of cola a year. If they do, God help the= m... Diabetes, tooth decay, digestive problems, caffeine addiction, pulmona= ry problems, etc. etc..
You may just have to punt this one and be content that your MacGuyver skill= s have produced a contraption that makes soda.