How much money would it take...?

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Let's say you are at the grocery store at an odd hour and so it is mostly empty. You see another customer drop some money on the ground way ahead of you at the other end of the aisle and then they proceed to the next without noticing. You go to the where the dropped money is and have to decide whether or not you will return the money or just pocket it. It is my belief that there is a certain limit that people who use to determine whether or not to chase the person down to give them their money back or to just keep it. For instance I doubt many people would bother to return a dropped nickel. Some maybe wouldn't even bother for a dollar bill. So my question is...how much money would it take for you to track down the stranger and return their money?