From "Atlas Shrugged," by Ayn Rand.
See, Ayn Rand's problem was that people aren't really like she said they are. She faced an additional difficulty in this case in that she was not a mother, and thus could not know what she was talking about. Rand apparently assumes that mothers will simply perform a cost-benefit analysis when their children are in danger.
Where the safety of their kids is concerned, however, most mothers don't even consider relative value. They will do anything to save their kids, no matter the cost to themselves.
So a scene where Lily Potter shields her child with her own body is completely believable. A scene where Lily Potter hesitates between Harry and "Ayn Rand's hat" (i.e., some deal with Voldemort) is not -- or if she does hesitate, we understand her hesitation to be wrong in some manner.
As so often happened when Rand's theories ran afoul of reality, this passage has her trying to force the square peg of motherly self-sacrifice into the round hole of her objectivist philosophy. And, as is typical with Rand, she is forced to make assumptions and assertions that don't stand up to scrutiny.