Yeah the CO2 bottles and exchange is expensive as hell. It's apparently cost effective if you use a commercial one but the outlay is pretty high. Not worth it either way IMHO. I just put up with flat shit.
There's also Sparkel, which uses a baking soda + citric acid powder in a separate reaction chamber. The cost is pretty low per bottle if you buy the bulk powders and mix it yourself; has been a while but when I had a Sparkel, I think I worked out that the cost was somewhere under 10 cents per 750ml bottle.