https://transitcosts.com/projects/ – a few cost under $50 million per km, a few cost over $1500 million

Here's there final report (424-page PDF lol): https://transitcosts.com/wp-content/uploads/TCP_Final_Report.pdf

They recommend that the State (note that, the State, not a contractor) appoints an experienced project manager who knows enough to know what the costs should be, and who controls the pursestrings.

They looked at high-speed rail as well as subways: https://transitcosts.com/high-speed-rail/