Or, to couch your question in other terms, is it like you have a p’tur that you choose not to use, (say its not suitable for a person of your stature to return such an object, yet you do so anyhow) where it seems likely there is a mitzvah, or is it that less than a pruta is worthless junk, and there would be no mitzvah.
Perhaps the sugya that suggests that a aveda that was a pruta at finding – even if it subsequently became worth less – is still required to be returned, would be a starting point.