These numbers are off - Mayo's salary averages $9.7m, Gronk $9m, Wilfork $8m, Talib $5m, Ninkovich about $3.5m. If these are this year's salaries, Amendola was only paid about $2m this year because his contract is backloaded.
That said, comparing salaries across positions is like comparing the price of tea with the price of oranges. Wide receivers cost a lot of money. For example, Mayo is the highest paid 4-3 OLB in football (and look at him! he didn't do anything this year after the first few games!), whereas the highest paid WR makes about $6m per year more. The highest paid RT in football only makes $7m per year, so as a percentage of that, the "injury-prone" Vollmer is far closer to the top than Amendola is.
To counter your argument, I've offered up players who make more or comparable salaries to Amendola on a yearly basis, yet you've never actually addressed this.
Players who make more and produced less or comparably this season include Dwayne Bowe, Percy Harvin, Miles Austin, Sidney Rice, Santonio Holmes, Roddy White, Greg Jennings, Mike Williams, Stevie Johnson, Brian Hartline, Reggie Wayne, and Josh Morgan - that is, nearly half of the players paid more than Amendola. Note that some of these players (Harvin, White, Williams, Austin, Johnson, Wayne) were injured for substantial portions of this year, but if we're not going to ignore that for Amendola, then we can't let those guys off the hook either.
Players who make within $1.5m of Amendola's yearly salary and produced comparably or less (again, disregarding injury) include Devin Hester, Michael Crabtree, Nate Burleson, Santana Moss, Nate Washington, and Lance Moore.
Amendola's compensation is perfectly fine.