Quote Originally Posted by yedmundi
The cheapest CO2 is simple... fresh air from outside... Baking soda and strong vinegar works pretty good too, and is cheap... Sugar is pretty expensive for the amount of co2 you get. You also get alcohol evaporating into the air, which isn't good for the plants.

Cheapest co2 would be to burn something that produces a clean burn, like propane. But only do that if you have sufficient cooling. You only need a small amount of co2, so keep it small.

When the plant stores energy, it needs co2. But when the plant uses energy, it needs oxygen. So don't produce so much co2 that you starve the plant of o2.

But that gets you back to fresh air is the best thing.
The cost ratio of sugar to produce Co2 is actually the cheapest, and your assumption that alcohol "evaporates into the air" is wrong. When yeast eats sugar is makes a molecule of co2, and a molecule of alcohol. Co2 is heavier than air so when it is released it creates a layer on top of the wash that blocks any alcohol from evaporating. Alcohol only starts to evaporate when the fermentation is complete, but even so it wouldn't be anywhere close to enough to harm plants. It would just be the equivalent of having an open jar of wine in the grow area.
Fresh air is great, but we are talking about the benefits of raising co2 concentrations for a grow. I don't think there is any question about it's effectiveness, just look around these forums you will always find positive results.