Krul has been impressive for Newcastle as the Magpies enjoyed an excellent first half to the season.
Chelsea and Tottenham are both rumoured to be interested in the 23-year-old Dutch stopper.
A Newcastle source told The Sun: "Tim is now widely recognised as one of the best goalkeepers in the Premier League.
"You've got to be talking £15m, maybe more. That's how highly we value him."
Alan Pardew's side are currently seventh in the Premier League.