I'm a Red Sox fan so I'm probably biased.
No, he is not worth that much, i don't believe any player is. He is a tremendous talent, but his tenure with NY proved he is not the end all to a championship.
Having the best player (offensively at least in this case) does not = championship.
So much goes into being a champion beyond having the best player.
In baseball, it just about always has to start with the pitching, then a good combo of offense-defense-good luck (as in no injuries to key players).
A-Rod may be the best player, but with that kind of money, you can go a long way toward building a "team", and a pretty good one at that.
Team chemistry is also an under-rated quality. Based on some media reports (which are not always accurate, I agree), A-Rod falls short in this area......despite his great on the feild skills.
Someone will pay him though, that's just the way it is now days.