There is one goal when you get robbed. That is to walk away from it. Yes, you want to minimize the damage. Minimizing the damage is a minor goal. The main goal is to survive. The robber with the gun has 2 goals. The robber wants to maximize his take of liquid assets (cash). The robber's other goal is to leave as quickly as possible. Notice, that at this point there is no conflict in your main goals.
If you have a gun, forget about it. The robber has a gun in hand with a finger on the trigger. If anything grabbing for your gun will lessen your chances of walking away. Follow instructions. Don't argue, or make counter offers. The quicker that this is over, the better the chances of everyone walking away. The question is not “What's in your wallet?”. The question is “Is it worth dying for?”.
I was robbed at gun point back in the eighties. The man had a clip board in hand when he approached me on the street in front of the consignee. He was offering his lumper services and I turned him down. At that point he pulled the gun from under his clip board. I vividly remember his hand shaking. That made me as nervous an anything. There is no bravery at this point. You have two choices. You can be brave, or you can survive. I handed him my wallet. He removed the cash from my wallet and handed the wallet back to me. Later, I realized that he knew that if he took the money and left me with my ID and credit cards that I would be less likely to call the police.
Interestingly enough there was another Wisconsinite that this robber tried to rob. The other Wisconsinite had seen the routine there before and had his gun in hand as the robber approached. He got the drop on the robber and the robber fled. He was talking it up when we were on the docks. I admired him. I lost $40 and that was a lot of money to me. Mind you, not enough to risk my life over, but a lot of money. This other Wisconsinite was cool. Then we walked out to our trucks. His driver side window was broken out. There was only one thing stolen from his truck. His gun was now on the streets of New York.
There are a few things you can do to reduce the damage. If you are in a questionable area, build a fake wallet. Put some cash in it. Put some fake credit cards in it. Grab a loyalty card at the truck stop and put one of those in it. Don't put anything traceable in it. Identity thieves are out there too. These days that robber may just keep your wallet and sell your credit cards. No matter what happens the main goal is to walk away. You can't be replaced. Everything in your wallet can.