There is No Chiyuv of Hashovas Aveidah for something worth less than a Perutah but is it still a Mitzvah or not?
The Nafkeh Minah would be – Oisek B’Mitzvah Patur Min HaMitzvah
Probably OBPMH only applies to a chiyuv
One stops learning if there is a miztvah that cannot be performed by another since the CHIYUV of talmud torah is only during free time so once the miztvah comes there is no OBPMH.
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.