A large sum would prompt me to report it. :yes
A small amount, say, $16.00, I would keep it.
A true story:
Several years ago, I made it to the train station, on the way to work, with about five minutes to spare. I went to the ticket vending machine and realized I did not have my wallet! The train was coming and I contemplated my next move. Do I go inside and explain to the ticket agent what had happened and ask to spot a round trip ticket until the next day? Do I get on the train and take my case to the conductor?

.
All of a sudden, the wind blew and blowing my way was a $10, a $5 and a $1. $16.00! I put my foot down on the money so it wouldn't blow past me. (amazingly, the money did not separate itself). I picked it up and looked around to see if anyone was looking around for lost money. I would have gladly given it back if they were.
There was no one around. I had just enough time to buy the ticket and board the train. I got my ticket, had enough money for lunch and still had $2.00 left when I got home.
I chalked it up to Divine intervention and thanked God while on the train headed for work.
