The cheapest way would be to use a CO2 generator, the expense and time to get it hooked up and working is somewhat prohibitive as it is needed for larger grows than most people have. If you aren't growing 50 pounds of dank then the best way is with CO2 bottles, regulator and solenoid valve. Effective PPM concentration is 1500-1600 not 700. Do not use during veg stage, we want the buds to grow big, not the leaves.
The homemade co2 generator of yeast and sugar does not provide enough CO2 to make the 1500 ppm, perhaps several of the rank smelling apparatus can be used , the smell will gag a maggot.
No one is going to rat you out for filling a 50 pound bottle twice a year, thats hardly noticeable, now if you are filling 100 bottles each month that might be something to worry about, but those kinds of grows use CO2 generators as the economy of scale then makes it practical.
Use of a propane gas stove will kill you faster with carbon MONoxide (CO) than the plants will benefit from the CO2. Do not use a camp stove indoors, thats not only a hazard from poisoning , but it also is a fire hazard.