Given how much he has improved and his versatility in defence I would say that he shouldn't be sold for any amount regardless of the final valuation whether its £10 or £25 million.
He gives 100%, can rampage down the wing till the last minute of the game and is hard to beat on a one to one in defence.
He does have some flaws and makes mistakes but I think that he is an exceptional talent who is constantly improving. If he keeps developing I think that he's going to be one of these players that can lift a team and a crowd when the going gets tough. He plays for the shirt and for a young player I feel confident when he's on the park. "Rangers through and through".