Floyd3 said "where the cheapest is, however do not just take the total price, check how much propane you are getting for your money. Many exchange tanks have less than 20 lbs of propane. "
All tanks will be filled less than 20 lbs because you only fill propane cylinders to 80% capacity to allow room for expansion so they don't explode.
So a 20 lbs tank should never have more than 16 lbs in it. This applies for exchange tanks and tanks you get refilled.