Why it makes sense to trade for him now as opposed to waiting for him to potentially (but probably not) become a ufa.
1)Control. - Likely (but not always) a verbal deal would be in place. Plus, even if a new deal was not immediately reached, having him on the roster and showing a player you want him and are willing to trade for him gives you a leg up in negotiations (plus there is an exclusive negotiating window). When waiting, upcoming high profile ufas quite often get tagged or signed before they hit the market. Additionally, the market is more competitive for a ufa than the trade market. This would likely be the acquiring team's one shot.
2) Finance -Teams make money. Finance is all about manipulating the cap. Assuming the team has the cap space (most would)Trading for AJ now will allow a team to re-sign him early, giving him the contract he wants, while spreading out the cap hit over an additional year.
3) Talent - This is a win now league, and if you are in a front office on thin ice and believe you have a playoff shot.... You might not get that luxury of bidding next year.
As far as the price, who knows. The injuries have put that all in to question, but a team would absolutely want him if he can get back on the field. He is by no means untradeable. (I hope and believe that he stays)