What would the cost be for the same distance in other parts of the world? Even accounting for inefficiencies, do you have any data on that? A quick search seems to put SF at the top [1].
In Stockholm there's a current ongoing expansion of the metro system, the blue line is being expanded towards Nacka to the east and to Bakarby on the northwest while connecting to the south on an expansion. A new line (yellow) is under construction, the green line is being expanded.
In total there are 18 new stations, digging under bodies of water (and quite deep through a lot of the blue line), total cost: around 5B€.
I’d assume that drilling a tunnel under a mountain would be significantly cheaper than digging one in a densely inhabited city (in a seismically active area).
[1] https://pedestrianobservations.com/2020/09/13/the-costs-of-s...