Tunnels are very, very expensive.

e.g. https://pedestrianobservations.com/2016/12/15/second-avenue-...

> The total length of the project, from 96th Street and 2nd Avenue to 125th Street and Lexington, is about 2.7 km. At $2.2 billion per km, this sets a new world record for subway construction costs, breaking that of the first phase of the same line, which only cost $1.7 billion per km.

I read one article that blamed the high cost of modern subways primarily on the hyper-elaborate modern subway stations and the actual tunnels are a small component of the cost.

I think you're right - tunnels seem to be about $200-400M per km when considered purely on their own.

Musk seems to think he can do it faster/cheaper. His track record makes it worth considering.

The article I linked to smacks his arrogance down over costs that he's overlooking, but even that may be addressable. Let's see what he delivers in the next 5 years.