I wouldn't see him costing more than 10m to be honest due to his lack of play time and injuries. He must be in his last year or two of his contract by now as well.
Edit - Just looked him up he has 2 years left with an option for an extra....
FYI though he has played 10 times in the Premier League and 30 times in the Championship he is 23 in November, he has played 3700 mins in his entire career competitively, which is the equivalent of about 38 games almost solely in the Championship with us. For me there is no way they could legitimately demand a massive fee for him and realistically get it.