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.