One other thing to consider is that there are several teams that are needing some major revamping and might be willing to spend more now than they've ever considered before. I think A-Rod could go anywhere he wants, as long as he's willing to take a pay-cut, and I think one place he'd highly consider would be the Angeles.
While I wouldn't oppose him being a Red Sox, I like the youth of the team and that they have room to improve even though they're pretty much set for the next 5-8 years. With one of the younger and most future-proof teams in the league, I think Boston would probably gain the least of any team when it comes to signing A-Rod. They really don't 'need' him and I can think of a lot of other teams that would be willing to fork over the cash that could use him more.
As far as the Marlins, I don't see them giving out that kind of cash with the lack of support they have. They obviously have stadium issues and I think A-Rod would rather go somewhere he could win within 2-3 years, if not right away.