Monetary value for an autograph depends mainly on 3 things:
1. The rarity/uniqueness of the signature aka does the person sign a lot of things?
2. The rarity of the item being signed.
3. The rarity of the person aka are they still alive and how famous are they?
In this case I would say the signature would not increase the value by any significant amount. Chris Evans is still alive and he signs a lot of items relating to Captain America. Also, I'm not sure having the shield itself signed was a better choice than the outer box art or the clear inner box. If it was one of those prop shields then yes because that is the whole item but the figure is the item and a collector who displays it might not want a signature when posing it with the accessories. If it were my nephew I would tell him to keep it.
Of course, this is all my opinion but I did watch a ton of pawn stars back in the day so I guess you could say I'm an expert.
