It will cost him $900,000 dollars, and also likely the $5.4 he was scheduled to earn next year (although it wouldn't surprise me if someone else ended up paying him comparable or even more money next year.)
So far he has been paid $11 million in 2009 ($5 million signing bonus and $6 million salary) and $23.7 million in 2010 ($21 million option bonus and the rest salary.)
The Redskins are going to try and reclaim some of the option bonus he was paid this year by saying that he has not performed services as agreed to in their contract. The Eagles did not have success with trying to do this (beyond 4 games) with T.O.; I believe the Falcons did have some success at recouping bonus money to Michael Vick.
I'm not sure exactly what bone you have to pick with Shanahan, but to suggest it's a show to cover his own shortcomings is just ridiculous. They paid Haynesworth a ****-ton of money, and they are positioning themselves to try and get back some. Additionally, there isn't a head coach in the league that would stand for the BS that Haynesworth has dealt out so far this season.