I'm not assuming anything _exactly_ (7, 8, 9 million may all be fair contracts; $12.5 million probably isn't). I just don't think that he's worth more than a healthy Kendrick Perkins (and a healthy Kendrick Perkins is probably worth his current contract). I hope that makes sense.
He's a much better rebounder than a never-been-injured Perkins would have been. I'd argue he is a better defender. Could Perkins make up the difference on offense?
Perkins posted defensive rebounding rates of 21, 24, and 26 (post-injury) in the seasons when he was 24, 25, and 26 years old. I focus on DRB since most of his work came in a Celtics system that specifically deemphasized offensive rebounding. At the same stage of his career, Asik posted 22, 25, and 31.
Asik is probably the better rebounder, but not by as much as you suggest. It's all academic at this point but there is an argument to be made that if Perkins were healthy in his age 26 season, and didn't have to subsequently lose weight to alleviate the pressure on his surgically repaired knee, they would have been equal rebounders.
Also, I see them as somewhat similar defensive players -- Asik perhaps with better mobility, Perkins the better shotblocker of the two.
Perkins also had a little better post moves and hands. Overall, though, they're largely the same player -- and it's worth noting that on a per-minute basis, Asik's best season is very similar to Perkins' best season.