Honestly I think I personally would just step over the money and keep walking... I actually did that the other day, to a dollar bill on the ground. I don't need, financially, to pick up dollar bills from the ground; maybe the next person who comes along does, and it will be great for them. Unless there's someone five feet further on, digging frantically in their purse, there's very little way to accurately determine who the cash belongs to--not by me, the police, a nearby store manager, etc.. And that's assuming everyone involved is completely honest and invested in returning the money. So basically it's a choice between me keeping money I know isn't mine, or me giving it to other people when I also know it isn't theirs (like a store manager). Or, you know, just stepping over the money and walking on.
Plus, honestly, I think I've just seen too many heist movies.
I watched a really lame one the other day where a guy found literally a suitcase full of cash--a hundred thousand dollars or whatever--on the side of the road... the same day as a bank robbery nearby... and he promptly decides to take it home and spend it with his wife upgrading their dingy lives, with this "miracle money." I was really kind of rooting for the robbery-associated hitman in the end... Granted, I don't think times are hard enough yet to send a hitman to retrieve a dollar bill, or even a hundred dollars.