That's not how the value of human life works... And abortion isn't comparable in any way to overpopulation, not even as an opposite. They are entirely different topics. Also, no one says overpopulation decreases the value of human life.
@donald.Keller - it is frightening notion to believe that overpopulation could cause a decrease in the value of human beings, but it is a fact that overpopulation leads to a drop in human rights. Imagine if the human population on the Earth was to be 25 Billion people; likely more than half of them would be starving to death. We may in our lofty ideals wish them to be of equal value (and I acknowledge that their suffering would be equal to that of anyone else's), but "values" change when stomachs are empty and basic needs aren't met.
While i disagree that overpopulation decreases the value of human life because that is illogical, if we let people get abortions maybe we could drlay overpopulation and be able to save many more lives in the future that are already born.