According to Brand Finance (who run the site BrandDirectory.com), the Mets were the 64th ranked sports brand worldwide for 2012.
The site rates the Mets with a Brand Value of $145 million, and a Brand Rating of BB+ (anything in the B-BBB range is considered average)
As a point of comparison, the Yankees ranked 10th globally with a Brand Value of $398 million and a Brand Rating of AA. The top NL club was the Cardinals at 37. Overall the Mets ranked 10th among all MLB franchises – just ahead of the Angels and behind the Reds (yes, the Reds).
(I’m sure Howard Megdal can probably sink his teeth into this a lot more than I can)
The top worldwide brand was Manchester United (not a shock) with a Brand Value of $853 million and a AAA+ Brand Rating.
The full tables (which can be segmented by country) can be found here