It didn't drop when he had an awful 15/16 season, why assume it would be affected by another average season? Plus Koeman is a very good defensive coach who will undoubtedly get better performances out of Stones than Martinez could manage, and even if Stones wants to leave, he'll also want to play, and get in the England team, so he's not going to stay on the sidelines sulking all year.
At the end of the day we want a player they want to keep and have no financial need to sell. We're almost certainly going to pay what they want.