Here’s a controversial opinion, turns out Willie Mitchell might be just as valuable to a team as Ilya Kovalchuk, however since what he does isn't pretty or manifests itself in statistics it is a sacrilege to suggest this. You can expand this in general to wingers with little impact on the game when they're not scoring (there are worse examples than Kovalchuk) vs shutdown defensemen who can make a first pass and aren't boneheaded with the puck on their stick but don't put a lot of points on board.
Since the part of the game that these type of wingers are really good at often manifests itself in either pretty dangles and points it is human nature to assume they have a bigger impact on the game as whole than they really do. A successful dangle or a quick look at statistics is very easy to notice, dangling when you have a better option open or not using your linemates as well as you could, not engaging or losing puck battles, poor positioning or lazy backchecking are often very subtle and forgiveable in human eyes when you just went through 2 guys (even if the end result is a turnover, a poor or a blocked shot or something else). This often leads to inflated contracts for these type of forwards.
The defensive defenseman is basically the opposite of this. The part of the game they are good at doesn't really manifests itself well in statistics nor is the game they play pretty to watch or marvel at the technical skills like stickhandling, obscene shots etc.. I believe in general their contracts represent their value more accurately (though I wouldn't lose any sleep if a guy like Mitchell made 5 million instead of 3.5 which is probably below his actual value), however I think there is still some room for improvement in establishing who the best defensive D-men are when comparing them between eachother.
It's pretty easy to say a statement like - this guy is a 30 goal scorer and justify 6 million per year without having to worry about being crucified despite the lack of his overall game (even if he bombs later), but there is nothing to hide behind when you're shelling out the money for a defensive-defenseman, it's only your opinion as to what he contributes to a team, no stats (although there has been progress in the advanced stats community) to base the value on, I do believe the GMs are much more conservative in handing out the money to the defensive D-men for this reason compared to goal scoring wingers.. For that reason I think defensive-defensemen on average receive contracts closer to their value than the goal-scoring type of wingers mentioned above do (Heatley, Kovalchuk, Semin, Kessel etc. well the Kessel contract isn't that bad).