Originally Posted by LizardKing

You never know, who this belongs to/what they were going to do with it.
Of course it is possible that if fell out of the wallet of some billionaire, but in most cases its a very big amount of money for the owner and they wanted to buy something special.

If you did not work for the money, you should return it.

I don't know US law, but where I am from you can give it to the police and if the owner does not show up, you can keep the money anyway.