cheapest isnt the best. frozen water bottles wont work for large plants. the cheapest way that will give you the best results takes some effort and injinuity on your part. you need a small AC unit, some PVC(or some sort of other piping) piping to pipe the air from the AC unit into the tank. some where before the air enters the tank you must rehumidify it tapping a small room humidifier into the pipe should work. the whole set up i could probably do for around $175USD and have a very effective set up. that would work for a fairly large grow chamber.