Honestly, I feel like it's time for Matsuzaka to cash in now. If he turns down the Sox's offer, he's going back to Japan and his $3 million/yr salary. If the Red Sox offer him $12 million/5 yrs, I think it's a deal he and Boras are going to accept.
It's not just about the money this time. Matsuzaka has been saying since high school how much he wants to pitch in the MLB. He has an oppurtunity to really make a name for Japanese baseball, especially in terms of pitching. Do you think he's going to throw that away because the Sox offered him $12 million/yr instead of $15 million/yr?
That would make Matsuzaka greedy in the eyes of the Japanese public, IMO, and would really cause his status to take a large hit. I think it's important for Matsuzaka to be a good citizen for Japan here in the MLB, and walking away from $3 million, especially when the salary he'd be getting is 4x what he's getting in Japan, would be disastrous for Matsuzaka.
The Red Sox will sign him. My guess is 4 years, $12-$14 mil a year, with an option for a fifth.