When you use the exchange places you typically get a tank with 15 pounds of propane in them. I work at such a place and get tired of people saying that we are ripping them off. It says right on the exchange station that the tank that you are getting has 15 pounds of propane in the tank. We are not selling you 20 pounds of propane, we are exchanging your tank with a different tank with 15 pounds.
I personally go to a refill station that charges me for the gallons of propane that they put in. This is much cheaper. I typically pay $10 to $15 for 20 pounds of propane.
Now people will be posting that they can only fill the tank to 80% so you can't get 20 pounds into the tank. The correct answer is that these are really tanks that would hold 25 pounds of propane if they were filled to the top. They can only be filled to 80% so your 25 pound tank is in effect a 20 pound.