The cheapest way is to get a water bottle, fill it half full of water, add 1/5th of sugar to the water, than add a 1/5th of yeast to the water, and about 20 minutes later you will have a co2 system that is good for about 48 hours, just shake when it needs more bubbles... I tried this the other day and my plant really a seemed to enjoy it so i will continue to use this method.
Be careful not to put too much yeast in, that could lead to an overflow or your grow room just smelling entirely of yeast...