Okay, so my dad just got a Sodastream machine and I have to say, this thing is absolutely amazing. It's semi-affordable, at least compared to what you would normally pay, and makes incredible soda.
Here is the sodastream
I've gotta say, it makes some incredible club soda, but you can also use syrup from places other than Sodastream themselves, as the only thing the machine really does is put the carbonation in the water, and it does a DAMN GOOD JOB at that.
So instead of using the stuff that Sodastream gives(which is crap), we decided to buy a 5 gallon bag of syrup. After doing some research it turns out for coke, we need to give it a 5.5:1 parts for mix ratio(5.5 is the water). We made some, and it ended up tasting BETTER than the 2 liters you would get for $2 each. The bag of syrup ended up costing $55, since my dad got a discount from Sam's Club.
Yeah, it does end up being pretty pricey, but if you drink a lot of soda or seltzer water you're going to end up saving good money(not to mention the convenience of having a soda maker at home).
The bag of syrup ends up making about 27 gallons of coke, and you could always get the syrup for your favorite drinks.
The only syrup that I reccomend to get from Soda Stream, is the red bull syrup. They really nailed that taste spot on, and you can make 50 cans worth of red bull for a sleazy $5(By comparison, 50 cans of red bull would normally end up costing you about $150).