Here is an easy way that I just read about in a book a few days ago. Take around a lb of sugar and mix it with enough water to dissolve it all in a empty milk jug or something like that. Then add a thing of yeast and then that will create CO2 slowly for almost no cost. You can put the lid on it and run a tube from there to your plant area if you want to also then. It isnt the best but in theory would work and is cheap. I am going to try on my grow setup.