We should be aiming to sell him for more than his market worth. That's because we don't need to sell him or want to sell him. We have previous rebuffing large offers for players, he's got years on his contract, we paid a hefty sum for him in the first place. We have all the strength.
Bale was never worth 80m or whatever it was, I don't think any player should be. Bony wasn't worth 30m, Sterling wasn't worth 50m. But if the team selling have the power and negotiate well then they will get a big fee.
Clubs wanting to buy him have to make it worth our while.