Short answer: yes, he's expected to be worth about $ 56 M, not $ 82.5.
Burnett signed a contract for $ 82.5 M over 5 years, 2009 to 2013. We've seen a year and a half of the contract, and we need to project several years in the future to get a good idea. So, let's start.
On the free agent market, you can expect players to earn about $ 4.5 M per win, over a replacement player (your usual 25th man). However, since 2013 dollars are not the same as 2009 dollars, we'll say a win is about $ 4.8 M(*).
In 2009, Burnett had gotten 3.1 WAR, and so far in 2010, he's accumulated 0.5 WAR, which is below his usual, and why I assume people are panicking. Overall, he's given the team about $ 17.3 M in value.
Now, we need to do the tricky bit of projecting him forward. At the beginning of the season, the Fans projected him to 3.2 WAR. We're 45% of the way through the season, so we'll assume he'll collect 1.8 WAR over the rest of the season, which is worth $ 8.4 M. So, he's up to $ 25.7 M.
Looking forward, we'll use the rule of thumb that pitchers lose about 0.5 WAR per season. Pitchers can get better or worse, but because of the risk of injury, it's a good estimate.
2009: 3.1 WAR, $ 14.9 M
2010: 2.3 WAR, $ 11.0 M
2011: 2.6 WAR, $ 12.5 M
2012: 2.1 WAR, $ 10.1 M
2013: 1.6 WAR, $ 7.7 M
Total: 11.7 WAR, $ 56.2 M
It goes to show, it's very rarely a good idea to give pitchers 5+ year deals. They were expecting someone who could give them 3.4 wins a year, and it doesn't seem he's quite that good, but the deal isn't that bad.
Winners of free agent deals are often struck by what's called the winner's curse. If every player makes their estimate of what a player is worth, and it goes to auction, by definition, the winner will have overestimated the value of the player. This is why it's hard to make great deals in free agency.
But, there are a lot of variables, a lot of uncertainty, and a lot of assumptions. It's easy to comment after the fact, and a lot can happen in 3.5 years, so we might have to revisit this one later.
(*) I don't want to go too far in the economics, but I'm assuming about 5% inflation, which is halfway between the usual 10% seen in previous years and 0% seen during this recession, used as an estimate. Because of this, the dollars listed on Fangraphs won't be exactly the same.