Ryan told BBC Radio Sheffield: "If anyone wants to take a Doncaster Rovers player, they're going to have to pay top dollar."
Rovers came 14th in the Championship - six years after being non-league.
Ryan is under no illusions about the interest in some of his players, calling Stock "our Rolls-Royce midfielder - the best midfielder in the Championship".
In February Rovers turned down a £1.5m bid from Birmingham City for former Manchester City man Mills.
Doncaster's league position was their highest finish in 50 years and Ryan said: "I think 14th is a great position for us but it could have been so much better, in the second half of the season we'd have been comfortably in the play-off positions."