The specific phrase means "a reduction in cost per unit when buying in bulk." But psychologically, it means "reduction in effort per child when raising in bulk."
In 2003, Hollywood rebooted for a new generation. Directed by Shawn Levy (who later did Night at the Museum ), the film shifted the premise entirely. This wasn't a period piece about efficiency experts. This was a modern farce. Cheaper By The Dozen
The "gimmick" of the family wasn't just the size—there were twelve children (eleven surviving infancy, but the book counts twelve). The gimmick was the parents. The specific phrase means "a reduction in cost
Along with his wife Lillian, a psychologist and engineer, Frank applied these industrial principles to their family of twelve children in Montclair, New Jersey. The book’s title refers to a favorite joke Frank would tell pedestrians who stared at his massive brood: when asked how he ended up with so many kids, he’d reply, "Well, they come cheaper by the dozen, you know". "Cheaper by the Dozen" and the Tyranny of Technology This was a modern farce