The 19-year-old, who competed in Stuart Pearce's Team GB side at the London Olympics, is expected to start for the Three Lions in their friendly against Italy tomorrow following the withdrawal of Manchester City shot-stopper Joe Hart.
"Has his stock risen? Of course," the Daily Star quotes Clark as saying. "He was one of the stand-out players for Team GB and if he does get a cap, it will only enhance that.
"I wouldn't want to entertain talk of selling him. But if you've got an international cap you go into a different league. £6m just gets you a League Two goalkeeper. A full international, you'd need to treble that."
Reports claimed in June that newly-promoted Premier League side Southampton have had a £6m bid for the goalkeeper rejected.