The idea of value being based of rarity is true in the economic field, but in the body, value is based on usefulness. For example, in freezing weather, the blood will move towards the heart and central organs. The body values these organs because without them it will die, and it's willing to sacrifice toes to keep them alive. The idea behind the rat-brain study is that the body is willing to go to greater lengths to keep the female neurons alive, therefore it must find them more useful in some unnamed way than the male neurons which it is more quick to sacrifice.