The tough part of this question is that I think it completely depends on the context of the rest of the roster.
For example, if they already have 2-3 stars locked up, and you are looking for a role player who can basically dominate on D, but can be hidden offensively, then I think there is an argument that he would be worth $8-10 million per year.
For example, if he was coming up for his extension in 2007, and the C's just got KG and Ray Allen, and they were still in their primes, you pay him what you need to pay him to keep him.
However, if your only star on the roster is Rajon Rondo, who is, generously, a third tier star in this league (which is certainly not a bad thing, but he is not Lebron James), and then a bunch of guys over the hill, who you are waiting for their contracts to run out, then I think it is a different situation.
Basically, that puts the C's in a position where they need to find a way to get at least 2 more star quality players before being anything close to a contender.
Most likely, you are looking at a minimum of 2-3 years where this team is struggling to make the playoffs.
In that situation, I do not think you can afford to give an All-Defense/no-offense player like Perk that kind of contract. All it would do is hinder your ability to get those stars that you need, and frankly, he will look significantly worse, since you will be asking so much more of him, and unable to hide his weaknesses, which then turns him into a negative asset in your attempt to rebuild, since his contract is blocking you, and no one will give value for him in a trade.
So ultimately, unless the C's are able to pull a miracle in the next 8 months, to bring in some younger stars, I think you need to try to move Perk for a player that has a better chance of building his value, or has a chance of being more than a role player.
...of course, if he is willing to sign for $5-6 million per year, forget all this, that is good value.