It's not unique to the AT&T Unlimited Data Plan. Apple and the carriers had a strategy early-on to gain market share by deeply discounting the iPhone, and Apple and the carriers have a strategy now to maximize profitability by increasing the price on the iPhone.
Anyone who had an iPhone pre-2014 is now being forced into a non-subsidized full-price model and it will cost us 100s if not 1000s of dollars.
Apple gets paid full price for their phones, always. Carriers used to deeply discount the phones to gain more loyal subscribers, now that they've got us locked up they are colluding to stop the discounts and make us pay what they are paying Apple. So you understand how this works before speaking, read:
http://www.wsj.com/articles/SB10001424052702304020104577384562576617618
BJ
How are you locked in? Switch to a different carrier.