A "low-cost" subway is generally figured at about $80-100M/km, and in many more countries, you'd figure more like $150-200M/km as a good number.

The Chinatown subway was $600M/km, which is 3-4× "reasonable" construction costs, and fairly far from the most expensive subway, which is I believe NYC's Second Ave phase 2, at almost $4000M/km.

The US in general is a high-cost construction country (for a variety of reasons), but relative to the rest of the US, California is neither particularly expensive nor particularly cheap.