you pay them up to a level where if things don't work out a player could be moved on, if you are paying them over what they could earn at another club, then its too much.
If we offered Dybala 300k a week he would be the joint second highest earner in the league behind Sanchez and level with Ozil.
Dybala is a really talanted player, but he hasn't quite become the world class player people thought he would a few years back, and can be quite inconsistent. Put a player like that, who has never played in the premier league, on a wage that would make him really hard to move on is not a good idea.