In other countries, the amount of rail you already have is not really indicative of stopping new rail construction any time soon. Seoul, Paris, Moscow, Beijing, Shanghai etc. all have very large metro networks and are not stopping any time soon.
The problem is not really dollar amounts but effective spending. East Side Access ended up costing $12B for one additional station and a few km of track, where some of the tunnels already existed. Consider that in Paris, they just opened a 15km extension with 8 stations for about 3B EUR.
You kind of point to the reason why its so expensive to build in the US: we stopped building. Any new projects are unique which requires special orders on everything. When you're constantly building and upgrading, your overall costs keep dropping.
There’s that but there is also a lot being spent on the wrong projects. East Side Access has not really moved the needle on ridership, which is a very big contrast compared to say Crossrail in London which was built in the same time frame.
There's also a lot being spent on things that are not even transit, but are required/customary of transit projects. The consultant-class has to get their 10 pounds of flesh, and every project is needled to death by outreach, impact studies, environmental reviews, delays, more environmental reviews, etc. We pay a lot more for the stuff, sure, but we've already blown half the spend before we've even started the job of putting stuff together.
This is of course an over-correction to an equally bad world where you can just get some dopes together to slice up your city like a Christmas ham, usually by demolishing the poor and non-white parts, to build highways.
The problem is not really dollar amounts but effective spending. East Side Access ended up costing $12B for one additional station and a few km of track, where some of the tunnels already existed. Consider that in Paris, they just opened a 15km extension with 8 stations for about 3B EUR.