$700+ or $1, depending on wether you go for the super-expensive system with tanks and regulators or a bottle full of fermenting sugar. Google DIY yeast Co2 system.
A good alternative to diy is the Nutrafin Natural Plant CO2 system. It's basically like a no-hassle DIY system and comes with a good diffuser. You can buy the sachets or eventually use yeast and bicarb to keep it going. You'll only need one for the the 10 or 20 gallon tank.