I have Walker on my team, and I've basically resigned myself to keeping him. Like mktgteach said, you won't get his potential's worth back.
When he's in there, he's amazing. He even throws in a few SB's! I'm riding him out until he gets hurt. He seems to have made it threw the elbow problem OK, so let's hope for the best!
I'd definitely put him in top 20, but he'd easily be top 10 if you didn't have to worry about injury. For a trade, though, you're probably looking at someone who's 20 to 30.
Best bet would be high risk/high reward for him. Maybe get Kearns and some other mediocre player? Or something similar, because you won't get a veteran who you'd be happy to give Walker up for.
He's frustrating because you always worry about him, but I'd rather have Walker than a lot of other outfielders!