I think it's one of those rare (though increasing common) transfers where if we are told it's £100m+, then we turn round and hand over the cash. I think he's that special. Not sure where it would leaves Foden and Grealish if he did come, but I think Mahrez would be accepting of a bit part role.
Trouble is I can easily see Napoli asking for over £150m, which is probably too much for us to justify whilst maintaining financial sustainability, unless we of course want to send a message to the rest of the footballing world.
Spending £500m or more on players is something many clubs can now do. Spending it on the right players however is something we still do better than anyone.
I would suggest however that anyone dealing with Napoli this summer goes early and goes big. You'd have to accept the figure on the table or move on. They have a few promising players who may be open to a move but don't need to sell more than one of them.