"Four-year, $24.935M extension" sounds worse than it is when you realize his rookie contract year is included. That extension makes him the fourth-highest paid center in the league over those four years, but the details are more team-friendly than that:
Slated to earn $565,000 in base salary in the fourth and final year of a $3.214 million rookie contract, Unger received a $5.5 million signing bonus and his base salary has increased to a fully guaranteed $2.5 million for the upcoming season. Unger will earn $4 million in fully guaranteed base salary in 2013 before earning $4.5 million (non-guaranteed) in both 2014 and 2015 and $4.25 million in base salary with a $250,000 roster bonus in 2016. Undisclosed base salary escalators are available from 2013-16.
Still a pretty steep contract.