Check garage sales. Some people who are relocating a long distance sell full or almost full 20 lb propane tanks for ridiculous prices. My neighbor purchases them when he finds them for under $1.00. He now has 8 almost full tanks in his storage shed. Several he purchased at garage sales for 25 cents each! He shared this information with me today. Saturday I go hunting!
I have a shed full too...I generally pay more...about five bucks each, some full some not.