There is no reliable information about In-park sales, except that 31% of all local revenue is given to MLB. Just by reading that I'm inclined to believe that would include in-park merch. Even so, how much revenue would be generated on in-park Mauer merch? Not as much as you would think. Just hypothetically (cause I'm bored waiting for my breakfast), how many jerseys do you think are sold in-park per game? 100? 1000? I've been to hundreds of pro games in my lifetime and never once bought a jersey at a game, Sure, I've bought other crap to appease the kids, but never a jersey. And I own probably a dozen or so jerseys (not all baseball). We've all bellied up to the stand that sells team merch, and I honestly can't recall seeing anyone buy a jersey. Ever.
Lets be enormously generous and say 500 Mauer jerseys are sold every game @ $100 a pop (just for easy math). That would generate $4.05m. Now give MLB their 31% and that leaves you with $2.8m, not chump change for sure, but not oil tycoon money either.
Point 2 - The Twins didn't HAVE to sign Mauer to appease the fans. In fact, they have a history of letting their star players go when the price gets too high (Knoblauch, Santana, Hunter etc). Now remember that 2010 was the first year of Target Field - they would have sold tickets regardless (which is supported by the above attendance numbers). Plus they already had his replacement on the roster in Ramos and if he was dealt to the Yanks (as rumored) I'm sure Montero would've been part of that deal. And that would leave plenty of money to sign a big-time FA (Holliday, Beltre, Lackey were available that year), in fact some wonks had Beltre signing with the Twins that year regardless. Or you wait until the off season and sign a guy like V-Mart or Beltre (also FA in 2011).
Say they signed one of those guys mentioned, I'm guessing the combined merch sales of Ramos/Montero/Free Agent X would've been far greater than Mauer sales, especially in 2011 when sucked dog balls, and possibly generated more ticket sales on top of it.
I know this is all speculation and "what if", but I'm not convinced the Twins had to keep Mauer. Last point, if this guy is truly the hometown hero type, why didn't he give the Twins a hometown discount? He doesn't know the Twins are a small-market team that really can't afford a $23m guy that misses 43 games/yr on average? He's an overpaid, overrated, douchebag that killed his hometown team.