FWIW...
I've shopped ATT rates without phone purchase. And recently. I review my options all the time.
Right now my line of service is $25 a month for my one phone. Or rather my one SIM which I swap between phones whenever I want. I have my own high speed data allotment. No "bucket" that others are dipping into fast as they can to burn up the use out from underneath me. And that $25 is flat. No taxes, fees or gotchas added in. I'm using the same ATT towers you are, so my coverage area matches yours. I can also roam. I spent 35 days recently driving around the western US and had service everywhere I went.
I've got 4 phones bundled on Cricket, each with 2.5GB high speed data allotment apiece. $100 flat.
For me to take those 4 phones to ATT I have to spend WAY more money every month for a bucket of data and each device has an access fee to the bucket on top of voice and SMS service!!!! The more phones I add at ATT, the more money I loose.
For just my phone, with less high speed data a month (2GB instead of 2,5GB) my rate as a user climbs from $25 a month to $55 a month. This costs me an extra $360 a dollars a year for cellular service. And not including the taxes, fees, etc. being added on.
There is no way my near mint (near only because it is has been used) 1520 sells for $360 dollars less than an ATT 1520 in the same condition with less features. That is like saying I have to pay someone to take my 1520. Nope. Incorrect and inaccurate.
$360 dollars a year is huge savings toward my next phone. And when I get a next phone I just buy it... I don't wait to sell the phone I have now. When you get a NEXT phone you never stop paying for it because they (ATT) have you signing back up all the time.