Luke Ball
Xavier Clarke was the player gained by having a priority pick. Not Ball.
Remember that even though PP's are put in before your first pick, that the club still picks with that pick the player it would have if no PP had of been given. So the players gained by being givena PP is actually the second player chosen and not the first.
So being given PPs gained St Kilda Kosi and Clarke and not Riewoldt and Ball.