The claim is usually that people are more productive in the office. Assuming that's true, then what's actually more expensive: having existing employees commut

The claim is usually that people are more productive in the office. Assuming that's true, then what's actually more expensive: having existing employees commute, or employing, say, 10% more people? Even if you only care about carbon costs, what are the carbon costs of raising, educating, training, and giving healthcare and everything else to 10% more people?