The 26-year old has become a target for some Premier League clubs and although Grayson admits an eventual move is likely, he believes it will come at a decent price and has urged his player to pick up his performances even further to get himself the best possible move.
"It would have to be a lot of money because we have turned down decent money already for him," Grayson said. "Jermaine wants to stay and finish the job off.
"If someone comes in with ridiculous money - and I'm talking a lot more than £2million - then we'd look at it again. But I've got no intention of selling him in the window because it's too late to get a replacement and he's too valuable to lose.
"Jermaine can only win because if he scores 15 to 20 goals until the end of the season he will be linked with bigger clubs than he's being linked with now."