First, if you make the concoction below, DO NOT pour sugar/yeast water on plants. You will KILL them. I have a thread going on this right now.
I've seen differing opinions on whether adding CO2 is of any benefit. That just breathing on plants will give you the same effect. On a dollars and cents benefit, I'd say no, don't bother with it. You are just speeding up the process by a day or two. My theory is nothing is too good for my plant, so I've been adding CO2. This is the recipe that I used (based on something I read on this or another forum.)
One clean 1 liter soda bottle. Add 2c sugar and 2c warm water. Shake. Put 1 tbsp baking soda in 2c cold water, add to bottle and shake. Add pack of yeast, but dont mix or shake. A rubber stopper with a hole in it for the soda bottle. [Note: it works better to dissolve the dry ingredients into water before pouring into bottle with a funnel. Otherwise you need a dry funnel and a wet funnel] Using some small flexible plastic aquarium tubing, run one tube into bottom of separate liter whiskey bottle half full of water. [I used this bottle because it had a wide mouth for three tubes] Top of whiskey bottle has two tubes going to plant. (don't put tubes going to plant in the water) All three tubes are caulked into top of whiskey bottle. One near top of plant and one in middle. Place several holes, an inch or two apart at end of tubes, so the CO2 is distributed to more parts of the plant. If its working, you'll see CO2 bubbling in the water, but very very slowly. Its suppose to last about 2 weeks. After a week you may not see any bubbling, just give it a gentle shake to mix up the solution.