Actually we don't exactly know what these so called bonuses work out financially, all the players who come here accept them, so I think on reaching certain goals I wouldn't be surprised to see these bonus reach a million or two more than their basic salary.
So in theory Vidal could be earning 6 million a year if we win the scudetto and he scores 10 goals. Which is actually a very competitive salary when the player has expressed his desire to stay.