The only time it pays to do an "exchange" is when the tank you are exchanging is expired or about to expire.. I've taken all my old tanks that were within a few months of expiring and exchanged for 'new' ones that were not more than 6 months to at most a year old...
Paid $20 and got a new tank that's good for another 11-12 years and got some gas in it at the same time.
Once those 'new' tanks empty, I just get them refilled at any station that does that..
Works for me.. You will pay over $30 for a new EMPTY tank in the store.. Then you have to fill it up!! You do the math.. :)
Good luck!
Mitch