I should add that I already have some 20# CO2 tanks... Buying them new is expensive... but I sorta inherited them from a good friend that knew I might use them if I started force carbonating Kegs back when I was Brewing.
The SodaStream CO2 refills are like $60 for two little cans...
Last time I filled up the CO2 it was under $100 for 4 tanks.
I have no clue how long one of those will last...
Their little cans of CO2 are 14.5 oz of gas and say they can do up to 60L of carbonated water...
If my math is right, one 20# cylinder should equal about 41-42 of their little refills.
(This doesn't account for the Refill exchange program which is something like 15 per refill plus shipping, but I would still have to buy a couple of their gas cylinders to take "advantage" of that program.)
(Thus, the only "cost" I really have is having a 20# CO2 tank on my kitchen counter!