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/
Having said that, Chart 4 shows that China isn't the best overall. It is better than average.Spain, Portugal, and South Korea have the most cost-effective systems.
I mean, I'd still take China over something IMF backed.