If you hand over a wallet with a reasonable amount of cash in it that appears to be a used wallet, why are they going to decide it's a fake? How many people really carry one?
* A mugger asks you to hand over all your goods.
* You hand him your fake wallet and cell phone.
* He decides to reach in your pockets to see if you're hiding anything else (maybe an iPod or something else).
* He finds your real wallet and now is aware that you tried to hide it.
* He gets angry and shoots you (whereas otherwise, he would have just run off with your stuff).
Is a slightly increased chance of getting beaten or killed really worth the difference in cash between the real and fake wallet? I think it's smarter to just prepare for the possibility of a lost/stolen wallet and know how to quickly cancel your cards and apply for new ones.