Clark feels recent events mean that Blues can demand significantly more money from any club aiming to take the keeper away from St Andrew's.
The 39-year-old manager said: "Has his stock risen? Of course. 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.
"Six million just gets you a League Two goalkeeper - a full international you'd need to treble that."