How much would a Willie Stargell signed winning 1979 world series hr baseball?

I think I am still up to date and the ball has not been found but if one did have it how much would it be worth and is there any way of proving it? Crazy grandpa claims to have got it and has ped on and we are still in possession of it. Claims to have it and signed in the 7th inning stretch by Stargell.