It is much cheaper to have your tanks refilled rather than exchanging. When exchanging tanks, you most always have a good amount of LP left in the tank. When exchanging tanks you are paying for the full tank, be it 20, 30, etc. lb tanks. Refilling you are paying for just the LP that had been used. Next time when removing your tanks, shake them and I'll bet you'll hear the liquid propane sloshing around. You could use tank heater to get more out so you won't be giving free LP to the exchange dealer to profit on.