Dayyyum, that is cheap compared to our 2 mile tunnel here in Seattle: https://en.wikipedia.org/wiki/State_Route_99_tunnel

$210 million a mile for rail is a great value compared to the cost to build here in the Puget Sound region. Sounds Transit 3 is adding 62 miles of rail for $53.8 billion and central portions like Ballard and West Seattle are still over a decade away from starting construction.

https://en.wikipedia.org/wiki/Sound_Transit#Sound_Transit_2