I really hope that A-Rod and Boras have over-estimated his value to teams. Looking for a 10-year, $300-350 million deal seems absurd, and I think most teams saw what it did to the Rangers several years back when he got the first big deal.
As for replacing him, I'm not sure who's out there at 3B. Names I've heard, in addition to the obvious Miguel Cabrera, are Hank Blalock, Joe Crede and Chipper Jones.
Given those options, I'd pass and try and deal for Tejada and move him to third. Cabrera is young and ultra-talented, but I don't know if he has the determination and drive you'd want from someone on your team. Girardi managing could help or hurt, but I don't know how they got along during Joe's one year down in Miami. I also don't know what Florida would want back, as Hank's made it clear that the "Big Three" aren't available:
ESPN - Yankees won't trade young pitching for Cabrera - MLB
Blalock is only 26, but seems to have peaked in 2004. His numbers have been declining ever since. Crede doesn't seem like a bad option, but he's only got one year left until free agency, and Boras is his agent so you know where that's heading. And Chipper is certainly in the twilight of his playing days.