It seems like, barring any ridiculous offers, Beltre will probably get an offer of 3 years at about $12-15 million a year. Originally, I didn't think that keeping Beltre would be a smart move for the Sox, but now I'm reconsidering that.
The primary concerns are that 1. Beltre performs much better in a contract year, 2. that the numbers he has this year are unsustainable, and 3. that a team will overpay for him.
1. I don't think that players perform any better in a contract year, IMO there are few people in baseball that would work a lot harder just because it was a contract year. Beltre had that monster year in 2004, but looking at his batted ball statistics, his LD, GB, and FB rates were all relatively similar to his career averages (19%, 41%, and 40% respectively; his averages are 19%, 42%, and 39%). So how did he hit 48 home runs? His HR/FB rate was 23.3%, which is just absurd. Basically, it doesn't seem like he performed any better, he just had a ridiculous amount of luck.
2. Beltre's power numbers are probably sustainable, but his average is probably not. His BABIP is .353, a lot higher than his career mark of .294; you could probably expect his BABIP to rise, given that he pulls to the Monster and will get more fly balls to go for homers in Fenway. I would imagine that his average would be at about the .290-.310 mark. His ISO is .233 this season, which is a touch above 06-08 numbers (which were about .200), but his HR/FB rate has been close to what he's done in past years (minus 2009, in which he was injured), meaning that his power has risen due to hitting stuff off the wall, and not a sudden rash of homers.
3. Will a team overpay for Beltre? I can't imagine many teams will, given the current economic situation and the reluctance to take on additional salary. Paying him $12M a year for three years doesn't seem unreasonable, although $15M is a bit high; but consider that the Mariners paid Beltre about $13M a year for 5 years. I doubt that the numbers will go much higher than $12M, and I think that Beltre would be worth that amount to the Sox given their need at the position.