The Scottish midfielder has been linked with Manchester United after a strong showing in the Premier League this season.
"The thing is, he did sign a new contract not so long ago. It'll cost a lot to prise him away from our football club.
"I don't know whether that price tag will scare people off or not. He's made big, big strides since he came last year.
"It'll be a massive price hike if someone does put the money down to buy him but I think he'd be worth every penny."