You cant keep it. If you do it is called "theft by finding" for which you can be charged. (Apart from the fact that it's not yours).
That's a tough one, if it was a drug dealers there's a risk if you keep it of getting your legs broken, but if you hand it in theres a chance you'll get it back as they won't go in and say they lost their drug bullion. But if the mad woman down the road lost it on a dementia fueled outing you'd feel pretty bad, but chances are she won't remember losing it What'sthe going rate for a reward these days? Is there a sliding scale of some kind?
Just take it to local police station and get some sort of receipt. Good for conscience, good for mental health!!