Butland was the subject of a reported £6million bid from Southampton in the last transfer window which was turned down by the Championship club.
That offer came before the 19-year-old England international had even appeared in a Blues shirt, having spent time on loan at League Two Cheltenham last season.
The youngster, who was included in Roy Hodgson's England squad for Euro 2012, has since established himself as Clark's first-choice keeper, notching up 22 appearances so far this season.
Premier League clubs including Liverpool and Everton have been linked with January swoops for the shot-stopper but Clark is keen to keep him at St Andrew's and says any bids will have to be significantly higher than in the summer if a club is to prise him away.
He told the Daily Mirror: "By the time the window opens he'll have played 25 or so Championship games.
"We would be looking for an even more substantial figure than we were offered in the summer - even though I certainly don't want it to come to that."
Clark added: "Ultimately, it's down to the board. He's the future and the type of player I'd rather build my team around."
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