Nobody's going to trade for Upton.
The reason why:
How do you value him? If he's just a rental, you don't give 3 top prospects for him, you work out a lesser package. If you have him for the next 4.5 years, and believe he'll more than earn his contract in WAR... than you offer an outstanding prospect package. Because you don't know WHICH way he's going to go, that's the risk. NOBODY is going to trade for him under this scenario because you don't know which prospects to trade. Trade your best, and he opts out, your team just got screwed. Avila, gets 1 good prospect and a couple flyers, and he opts in, the Tigers just got robbed. Impossible situation to be in....
The only way out of that scenario is for Upton to agree to opt-in or opt-out immediately upon the trade, depending on which way the buying team wants to go (and the prospects offered...)