An extreme example: 1 mile of subway in San Francisco cost just over 1 billion dollars to build. The central subway was a 1.7 mile long subway extension that recently finished construction:

"Total cost overrun, largely driven by the claims settlement, three change order omnibus packages, contingency reserves and additional project management and other services, puts the final cost at $1.89 billion and sticks SFMTA with a $184 million deficit for this single project alone."

https://www.sfexaminer.com/news/major-construction-on-centra...