I think by the time the trade deadline arrives, his contract value is roughly 1/3. So you only need a team to have about 7Mil available. Plus you add 5M and now you're only 2M away from a deal with an owner who is already at the limit of his cap. If he has ANY cap space that deal isn't hard to pull off. Also remember this season he is only making 14.5, so now you are talking 5M at the deadline which can be 100% covered via cash.
Now the 4th season at 20M might be a bigger problem in the trade screen, but ultimately I'd think you could pull it off with over half the league. Just need to hope the guy who wants him is in the half that can afford him.
Mike does make a really good point about the fact that many teams who are 2nd or 3rd best and feel they need that one impact bat may not have much at all on the farm to offer for him.
In one of my worlds we just had a team who was right on the cusp, with a CY Young caliber P1, a great lead-off man and a great Power bat, who felt he wanted one more great bat to put him over the top. He gave up some really good players including a pretty solid P prospect. Of course that guy was only making 9M, so yeah it gets tougher with the 20M guys.
I'll repeat my stance on only moving him if you get a REALLY nice package for him, and if by the deadline you are really short of expectations, because I believe the guy will be productive throughout the life of the contract, even though he is already beginning to decline. Increase your training budget and with that high of a make-up his decline shouldn't be devastating.