Anton Dubrau does a fascinating overview of existing tunnel boring machine technology:
People tend to think it’s the tunnels that are the most expensive part of underground systems like metros, but thanks to the already existing TBM technology, they often represent only a small portion of the overall cost. Sometimes as little as 10%.
The most expensive parts of subways are the stations. In my view, modern boring technology becomes interesting when we can use it not just for the tunnels in-between the stations, but to build the complete system, including the stations, cheaper.