I never really gave it much thought, which is weird because I'm a little cynical by nature, but my step dad always said they owed you 16-17 lbs when you fill a tank. I just switched to a propane smoker so I started paying attention. I have been filling mine at an Ace Hardware and usually come home with a little over 17 lbs of gas once you take away the tare weight of the tank. They charge $20 to take it to that weight no matter where it was when you took it in.
Swapping tanks around here cost between $17-20. I guess it would be better to swap a light tank and better to have an empty one refilled.