He's easily a 30m player in today's market, even with a year left, at his age. And clubs like Bayern should not have an issue paying that.
You are possibly right, in that the club won't put that over letting him go where he wants to though. I would still say it will be over 15m.
Doesn't send a good message to other players (like bernardo), letting someone else go for pennies, when the club consistently say bring the right offer and you can go.
We got decent and fair value for Sterling, Jesus, Zinchenko, Torres, without standing in their way, so I'd think we will for Walker too.