15m is too low for Maignan. He's still starter for the French NT, good age, experienced and a leader. Even if he has only one year on his contract remaining, he is worth more than 15m, we have signed Musah, Chukweze and others for +20m with a year remaining on their previous contracts.
The way I look about sales is.. would I pay 15m to sign Maignan if he were with another club? The answer is YES.
I would accept to sell if the answer was no. If they offered 30m, which I think is a fair value, then I would accept to sell him, otherwise better to keep him and run down his contract.
This applies to the other players as well.