I think it was a combination of this, and the fact that llorente was simply not working under allegri for whatever reason.
I think we really upgraded over llorente with zaza...so we can say that extra money is justifiable. He is more direct, bullies a lot (llorente didnt and we he tried he was a foul machine)... and Zaza has one thing that llorente really missed....
zaza has the hunger of master Quagliarella. Since Zaza knows his playtime is limited, he goes in with hunger to score even if that means shafting all the other juve players from goals. Thats what you need from an impact player.
The problem is, Zaza is not here to be a 4th striker, while llorente would have accepted the role most likely.