When I first came to London, when I was about 20, I had £70 cash in a home-made money belt under my clothes. It was all the money I had. I did not have a bank account at that time, and was struggling to do pub work. I lost that £70 in nightclub - ok, fair enough, night club - but I was in tears, asking everyone, and no one said they had found it. About 50 people or so, we're not talking 2000, it was a small place.
I find fivers all the time. Maybe I look at the ground too much. Especially in bars. People don't realise they drop cash.
Yesterday I found £10 at a small private party in a bar, only about 15 people. I should have left it where it lay, but picked it up, holding it and intending to ask if someone had lost it. And if not, then donate to the food or something. But in fact about 5 minutes later, someone called out, "has anyone found my tenner that I dropped?" and of course it was returned.
Not sure at all what point I'm trying make here... it just depends on where you are, what the situation is. Most people have lost money in their lives. Consider it just a bit of payback when you find some.
(I've also lost 3 digital cameras, all never handed in to any police station, 2 lost while cycling, all police stations checked... why would someone NOT hand in a digital camera? But I guess most people don't even hand in cameras, let alone cash.)