Personally I wouldn't sell him for anything under £10 million, even for anything below £15 million I'd be tempted to keep him.
Watching him against Cardiff proved how valuable he is to us and I think he's by far our best wing-back, perhaps Gibbs is up there too but he's too injury prone which excludes him from comparing the two.
I just think on/off the ball he is so much better than our others. Having said that, I know he hasn't played much recently which may be due to the transfer speculation which is not good.