Exchanging your old tank is suitable if you only want to keep one small unit. Otherwise, you might choose to pay the extra money for one you can refill on your own. Homeowners who maintain several small tanks or have easy access to a propane company may prefer this option. Buying a 15- to 20-pound tank you can exchange at the store could cost as much as $30 less than buying the same size outright. Exchanging an empty tank for a full one ranges from $10 to $25.