I went on cellunlocker site yesterday and priced the att unlock. It was $30. But make sure you activate with the original carrier before unlocking and using different sim. There is a new thing called REGION lock which samsung has started using. If you unlock before activation with the original carrier you will not be able to use your phone internationally. Cell unlocker explains it pretty well with a disclaimer about their service.
They say it applies to all USA phones released with 4.4 kitKat and all Samsung newer phones. The mega isn't specified, but all other top samsung galaxy phones are.
@ randy, thank you for your service and stay safe.