$100 [£50]? I'd more than likely hand it in to the Police, but if the money was a bit less than that, and depending on the circumstances of how I found it, I'd keep it. If the money was inside a purse or wallet etc, I would without a doubt hand it in, or if it was near other items, I'd hand it in. I worked in a hotel, one morning after about 3 hotel functions all on at the same time, I was on night porter duties. I walked around the function rooms after everyone had gone, and picking up £1, £5, £10, £20 notes here and there, I think I made about £800 that night and I kept it for myself. It seemed to just belong to random people that dropped it here and there. I also found money inside purses and wallets, and I found other niknaks too, and I handed them in to the hotel.
I found a watch once, handed it in to the Police, and 6 months later, they came to me and gave me the watch because no one claimed it. If I had kept it, and someone claimed it, and saw me with it, I could be done for 'Reset' also known as 'Theft by finding.'