Whether there is a "net benefit" depends on how much time they're in prison for, how much harm they caused before going in, and what kind of good they do/value

Whether there is a "net benefit" depends on how much time they're in prison for, how much harm they caused before going in, and what kind of good they do/value they contribute once they're out. Even after the time it would take to establish an effective metric, that's something that would vary very widely by individual.