Ok. Give up I guess; bad idea.
No. Let's start with this one, fix the various problems that arise, discover how to make a good paved road with the least amount of labor and resources, and continue to expand the paved network to reach everywhere in the city.
This is what happened. Would you want to go back to unpaved roads in a city? Traffic congestion is killing American society. I like American society. Many people don't like to live all packed together but still want to work with other people and have the amenities that cities enable. Cars and interstate roads allowed that for a few decades but now we need a new system to continue expansion of the free flow and interaction of people. Underground roads could replace interstates and we could turn them into grand boulevards for pedestrians, bikes, and other slow vehicles. Wouldn't that be great? Almost all car traffic is underground. Would you support such an idea?
The actual cost comes in station digging; stations need to be high throughput and extremely reliable, which favors complexes with a large footprint and thus more required property taking, usually in dense urban areas that require complicated engineering. For highway tunnels the similar cost factor are onramps and offramps; if you want a ramp to be safe and efficient at high speed you need a lot of space.
Hyperloop “solves” this problem by proposing elevators. Very few metro stations are elevator only because they are low throughput and are prone to mechanical failure, as opposed to staircases that are not.
An interesting writeup on the case study of the Second Avenue Subway: https://pedestrianobservations.com/2018/01/27/construction-c...