Ah - finally a topic I can claim expertise on. This is the MMM way of doing Sodastream.
1) Purchase a used 20LB CO2 tank which has a dip tubed installed. Go as cheap as possible here, you are going to be exchanging this tank out with your local welder, so just need something to get in the door.
2) Purchase this:
http://co2doctor.com/fillstationproIIspec.htm (make sure to either buy the paintball tank from here also, or a valve to trade out on your soda stream tank)
...Now $15/refill on a 20LB CO2 from welder shop buys you several years worth of C02 (most non-heavy users)... and you can refill as you desire.
3) Purchase this:
http://www.amazon.com/Valve-Coca-Cola-Brand-Bag-Syrups/dp/B009B1PA90 (taps into bag-in-box products)
Bottom line:
This was the cheapest way of doing soda stream. Most local sams club sell COKE Bag-in-box (BIB) soda's... PROTIPS = Refrigerate both the box of syrup and the soda-stream bottles, prior to to carbonation/mixing. The key to getting water to hold carbonation = low temp. The key to getting syrup to hold carbination = low temp... So you want them both cold. 5 to 1 ratio = Coke's official ratio. Draw lines on my bottle with a sharpie, to know where to refill water first, then syrup to. I used tap water, and accumulated a taste for it very quickly.
PROCESS = fill bottle with tap water (up to water line), refrigerate until full cold... when you want soda, take bottle, carbonate, then fill up to syrup line... creating 5 to 1 ratio.
Update = Although we've mostly stopped drinking soda because it cost extra and it's terrible for health, we still use this rig to make carbonated lemonade and home made sprite, with lemon, limes, ehthyriol & stevia sweetner.