There’s no doubt that it’s expensive to tunnel below the earth to expand the subway system. However, you haven’t looked at long-term benefits of using the subway. I’m the wrong person to ask because I’m not an engineer. But it’s pretty safe to say that it’s far more efficient to remove thousands of drivers from our highways and bring them all underneath the earth for transportation. Obviously, there will be less smog as well.
If that were true underground transportation would be cheaper. In reality it's the most expensive / energy consuming option and is only taken when there are no others.