I think I found a $20 bill once, long ago. Besides that, a $5.
When I was in my 20s, in NYC, I was crossing 7th Avenue (a wide, multi-laned street) at rush hour, when I saw a wallet packed with money fall out of a guy's pocket. I grabbed it and ran to catch up with him, but it took half a block before I did. Now, meanwhile, I'm waving his wallet and yelling, but you know New Yorkers - nothing. So I finally get to him as he's waiting for a Walk light and get there just as it changes to Walk. I grabbed his arm, because he wasn't responding even when I was calling him from right beside him.
When I finally get his attention, I'm holding the wallet out to him and he has the nerve to accuse me of taking it in the first place! He snatches it from me and I only had time to yell after him that he'd dropped it in the middle of 7th Ave. He never even thanked me, just rushed away.
At that point I was working full time and earning very little. I didn't expect a monetary award, but when he was so nasty, I really wished I'd just kept it. In fact, I felt like a fool for not doing so. Still do.
Well, what goes around comes around. A few years later, in Brooklyn (on the border between Borough Park & Flatbush, for those who care), I was the one who dropped my wallet, on 17th Avenue. A guy in a passing car slowed and honked and honked at me, calling to me. I thought he was yelling because he wanted me to get out of his way, but once I got across, he was still yelling and calling.
I turned around and gave him what-for for continuing to chew me out after I'd gotten out of his way, and I can swear a blue streak. So there I was... and the poor guy is pointing to my wallet (which he couldn't reach from his car) and I felt soooo bad! At least I apologized and thanked him.
Still waiting for my ship to come in...