If we could build subways for about of half the current low price by world standards (Spain or Turkey) cost most cities would have subways. However since New York "insists" on paying about 7x what Spain would for the same amount of subway it is just barely worth it to build the SAS, and anything with even slightly less ridership potential is a waste of money.

I have seen a lot of explanations of why the US pays so much more - they tend to not hold up. Note that last time congress asked the question the bill did not allow examining non-US construction for the report - something to write your congressman about.