All these phone company stocks are going down as people upgrade half or less as much as before IMO.
Were you on the NEXT program or on contract? If you were on NEXT, they would not have quoted you a price like that. I was on the NEXT Program, took my iPhone 6 in, gave it back to them, and walked out with my new iPhone 6s. My payments did not change. Done.
Image grabbed January 8, 2016, 12:12AM from AT&T Website.
The current NEXT minimum monthly pricing of iPhone 6s and 6s Plus on AT&T
So, if you had been on the NEXT program you could have upgrade at a reasonable price instead of having to pay an early termination fee, which you agreed to do when you signed the two year contract when you paid your downpayment for paying full RETAIL for your iPhone 6. . . which is what you did. You did not "buy" your iPhone 6 for $199 or $299. That was merely the downpayment on the phone. The contract specified if you terminated the contract early, you had to pay it off.
You could have paid the difference, kept the iPhone 6, had them unlock it as it would now be yours free and clear, and sold it on eBay where 16GB models are going for anywhere from $400 to $550 depending on condition.