The rationale this AM I heard on Boston radio is that the Red Sox are now in the driver's seat. Boras does not have any more leverage with Matsuzaka, in theory, because he can't pit teams against each other (a la "Bowling for Zito"). Therefore the Sox can set a price- maybe even below market. Matsuzaka made "only" $3 million last season for Seibu. Even if the Sox double his salary, they get a relative bargain. And word is Daisuke does NOT want to go back to Japan next season (he would dishonor himself after indirectly declaring his desire for better competition in MLB).
I say the final number ends up splitting the difference between the $12mil per Mussina signed for and the $6 mil that would double Matsuzaka's salary from Japan.
Call it an even $9 million per over 4 years. Maybe 5 if the Sox want to commit that long... heck, he's only 26 now- he'd still be just 31.