How much is a helmet worth when signed by...?

i have a patriots helmet signed by Randy Moss, Wes Welker, Adam Vinatieri, Kevin FUalk, Jerod MAyo, Brandon Meriweather, Tully Banta-Cain,Rob Gronkowski, Brandon Tate, and the owner Robert Kraft? i havent been able to price anything becuase i cant find any helmet that Kraft has signed, hes the hardest signature to get next to Bill Belichick. anyone have any idea what this helmet could be worth?