Meanwhile Switzerland, one of the top most expensive countries in the world, has tunnelled at the base below the alps (one of the most difficult terrains to tunnel due to vertical sediment layers) for ~20B. Workers died on that project too, but that risk is simply being managed rather than grinding everything to a halt.
I'd argue it's absolutely possible to improve the cost on American public transit infrastructure by one to two orders of magnitude if you can simply get a way around government capture.
While American construction costs do seem to run higher than comparative costs in Europe and Asia, the multiplier is closer to something around 2-3×, not 10-100×.
The cost of the Gotthard Base Tunnel comes out to around $300-400 million / km. By comparison, the Second Avenue Subway comes out to around $800-1000 million / km (I'm subtracting a bit because the subway has three stops--which tend to be really expensive money pits--and the base tunnel does not).
I'd argue it's absolutely possible to improve the cost on American public transit infrastructure by one to two orders of magnitude if you can simply get a way around government capture.