He was already rated extremely high when he joined us on the base of his last season at Gladbach (voted Player of the season by the players and coaches that season). Outside his last season, which was plagued by injuries, he scored at least 35 times in three years in a row with his career high being 46 in his strongest season 2013/2014. Over this time frame he averaged a scorer/minute ratio of around 90 which means that he statistically scored every game.
In terms of being a game changer, he was that even when he was not our overall strongest performer and most profilic player. He was an expert of either assisting or scoring the 1:0, which was even more important under Klopp than usual, because it forced the opponents to give up their defensive posture and allowed us to play the reactional style, which suited us the most. He also carried our game in quite a lot of games over the years, with the most prominent example being the return leg vs. Real Madrid in 2013/2014, when he singlehandily nearly caused a sensation of turning a 0-3 loss in the first leg into the elimination of the later CL winner.
In todays market he is certainly worth that much, although I don´t see us letting him go for that before 2018 unless he pulls a De Bruyne and forces his way out.