hey i dont know if its true or not, but i read it in hightimes that extra co2 wont be used unless there are more than 40 watts a square foot per plant. also 250- 300 ppm co2 is like regular every day air, to supplement you can use up to 1500 ppm to get the advantages. no i dont know of a cheap/reliable way to do it, there is the yeast method, or vinegar/ baking soda , or tanks with a regulator, also a fossil fuel burning area heater which will raise temps.