It seems like every time I read about a metro line outside the United States, except in the UK, it is way cheaper than we can do. ... Alon Levy has contrasted the cost of subway construction in New York with the much lower costs in Tokyo, for example. We seem to have a system in the US that significantly inflates the cost of construction vs. the rest of the world. Many of the typical complaints as to why this might be would seem to have no merit. Other countries are heavily unionized and regulated, for example, so don’t blame organized labor. (South Korean unions are famously militant). Spain and Japan are not exactly low cost countries. And basically all new systems world are fully compliant with equivalents to the [Americans with Disabilities Act].
-- Aaron Renn, the Urbanophile
I keep hearing this observation about US construction costs. It's totally outside my expertise, but if anyone has seen a satisfactory explanation of why US transit construction is so expensive, please link to it in a comment.