Great question. Here's one way of answering it. WHO will be in the mix for AB when all is said and done? Will it be well-run and successful franchises like New England, New Orleans, Seattle? Or will it be poorly run franchises run by egotistical owners that run their franchise like a real-life fantasy team?
Given AB's age, mileage, mental makeup, and contract, I would argue his trade value is actually less than a third round pick. He's got plenty of years left, don't get me wrong, but he is exiting his prime and already gotten the biggest contract he is ever going to get. For somebody to be able to fit him in the cap and their offensive system and locker room, it's not like we're talking about every team in the league.
At the end of the day, market value is whatever someone is willing to pay. In this case, whoever that is will be a sucker like Dan Snyder or Jerry Jones.
The Steelers scored big, got the best years out of AB, and many of them at well below market rates. Anything you get from a trade is a pure bonus, so don't look at it like not getting enough, look at it like getting away with robbing a bank.