The 19-year-old Blues goalkeeper has already been the subject of a £6m bid from Southampton earlier in the summer but the player's stock has risen since then.
After travelling to Euro 2012, Butland played for Team GB at the Olympic Games and could now make his England debut in Wednesday evening's friendly against Italy in Berne.
This is despite not yet playing in a competitive game for Birmingham, with all his experience of club football coming on loan at League Two side Cheltenham Town.
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."