There's another factor that's not mentioned here: the US has some bonkers expensive station design.
Take the newish Hudson Yards station. Its platform is about twice as wide as the original subway station platforms, and it's far longer than it needs to be too. On top of that, it has a full-length underground mezzanine above the platform, which is unnecessary. Overall, the effect when I first used it was "why is this so unnecessarily massive?"
Oversized stations are even more problematic when you consider that the MTA wants to build all of these by mining rather than using cut-and-cover, because cut-and-cover has too much disruption to surface tenants. And mining is far more expensive than cut-and-cover, so the extra volume you need to dig out for excessive station sizes hits your purse even faster. For as high as the actual tunneling costs are, it's the station costs that truly make the subway extensions stratospherically expensive.
Take the newish Hudson Yards station. Its platform is about twice as wide as the original subway station platforms, and it's far longer than it needs to be too. On top of that, it has a full-length underground mezzanine above the platform, which is unnecessary. Overall, the effect when I first used it was "why is this so unnecessarily massive?"
Oversized stations are even more problematic when you consider that the MTA wants to build all of these by mining rather than using cut-and-cover, because cut-and-cover has too much disruption to surface tenants. And mining is far more expensive than cut-and-cover, so the extra volume you need to dig out for excessive station sizes hits your purse even faster. For as high as the actual tunneling costs are, it's the station costs that truly make the subway extensions stratospherically expensive.