Depends on the situation. A few years ago I found a wallet in Las Vegas at a table in a public space. Clearly, that belonged to somebody, so I waited for them to return, which they did. If they hadn't, that's something you should turn into the police.
That being said, random money found in the road is yours. I'd say $100 is the cutoff, for me. Anything more should be turned in.
I'll never forget the time I was 8 years old and found a $50 bill on the floor at the grocery store. Thinking I was doing the right thing, I turned it into the cashier, who promptly put it in her pocket. Lesson learned.
If it's clearly not connected to somebody, and it's less than $100, it's finders keepers. Do with it what you please.