My Cook/Elphick comment was about 'selling players too soon'. Not attached to the future sell on percentage(Re:Walton).
I'm certain the Cook agreement for a future sell on is for a decent amount, considering the transfer fee to Bournemouth was next to nothing(£200K?).
If we did make the mistake of selling Walton for a sizeable fee, we would have to make sure our sell on percentage for his likely next transfer was a big slice of the cake. Hopefully we've learnt a lot from the Cook/Elphick situation and selling our young prospects too soon.