When I was a kid (high school age) I went out to cut the front lawn and found a switchblade in the grass out near the curb. This was in a quiet residential suburban community. Very little crime, mostly elderly folks (at least back then)...I couldnt have been happier! I always assumed that someone was in a car trying to dodge the police down a side street and tossed the knife out the window.
It was later taken away by a worker on a nearby construction site a few months later. Guy saw me and a friend playing with the knife, he came over, took it, and said we shouldnt have it. He made out well, from what I remember it was a good quality switchblade.
Also found a 12GA live shell in a field I was playing ball in.
As a kid I always loved shooting, I knew exactly what it was the second I saw it laying there in the grass, and I took it home and kept it on my shelf (my ammo display back then) for many years. As a kid, it was a treasured find.
Most expensive thing I found was a diamond pendant, in the mud, by the waters edge of a local fishing hole. I was casting from the shore and saw something shiny on the ground. Took a closer look and picked it up. Figured it was just a cheap pendant at first, but the metal was fairly solid and then I saw that it was 14k gold....so I put it in my pocket and kept it. Cleaned it up when I got home and realized that the stones really sparkled, so I figured it was worth taking to the local jewlery store and see the deal....sure enough, 7 real diamonds. They offered me $150 for it but I decided to hang on to it for a good luck charm. Eventually had it redesigned into a necklace for my wife...she still wears it all the time