Interesting take from Mark Zuckerman. He doesn't seem to think a deal will happen.
http://www.natsinsider.com/2012/01/addressing-fielder-rumors.html
This bit from Mark Z's piece is exactly on point as to why it either won't happen, or if it does happen, that the chances of Zim getting re-signed go down dramatically (or payroll goes up to $150 mil which seems pretty unlikely):
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Consider that the Nationals already are paying Werth $20 million-plus in 2014, 2015, 2016 and 2017. Fielder would also cost that much, perhaps has much as $25 million per season over that same span. Zimmerman figures to command $20 million per season himself over at least seven or eight years. Do the math. Are the Nationals willing to devote $65 million a year to only three players for at least four years?
Guess how many MLB clubs currently do that? Three. The Yankees are committing anywhere from $68 million to $72 million in 2014, 2015 and 2016 to Alex Rodriguez, CC Sabathia and Mark Teixeira. The Phillies will pay $65 million to Cliff Lee, Roy Halladay and Ryan Howard in 2013 (but only 2013). And the Angels are on the hook for $65 million in 2016 (but only 2016) to Albert Pujols, C.J. Wilson and Jered Weaver.
That's it. The Red Sox don't have three players making $20 million per season at the same time. Neither do the Rangers, the Tigers, the Cubs, the Cardinals or the Dodgers.