The truth is no one really knows that. It depends on the player. I've seen Hulk a lot and I would give Porto 30-35m with my eyes closed. However, knowing what Porto are like they will want more than that. It gets to the stage where looking at other alternatives wouldn't be a bad idea. Antoine Griezmann is a cracking little player for Real Sociedad, who's release clause is around 25m euros, but will likely go for less than that. He is younger and has great potential.