I formally exchanged tanks with the Costco in my neighborhood. They don't fill propane tanks, they only exchange them. I took in my empty tank and they sold me a refilled tank. Costco has a supplier that does this refilling operation for them. I thought I was getting a good deal until I discovered their tanks were only a quarter full.
I have my tanks filled at a large suppliers yard. I don't know what they are currently charging but it has always seemed inexpensive to me.
Just for fun I'll relate an experience I had with a tenant. He had passed away and his widow was cleaning out the stuff he had collected and stored in the garage. She didn't know what to do with the 40 (rusted, old style valve) full propane tanks. Naturally, I was upset with the potential bomb stored in my garage.
I talked with Costco and they said sure they would take them and I trucked them to their store. Their supplier now has replaced the valves and re-painted them. (Costco will take an old style valve, and the exchange tank you will receive in return will have the new style valve at no additional charge to you).
What a relief to get them out of my garage!