For those interested in a real deep dive, and I mean, deep, check out the blog https://pedestrianobservations.com/construction-costs/. And the comments are super informative!
It makes a lot of complex analyses using the best cost information available, and compares projects across countries, states, geography, politics, etc. I'm impressed by the technical engineering analyses. We IT people could learn a thing or two. ;)
Like, "this subway project in NYC cost $N billion USD, which cannot be compared to this subway project in France, which cost $M billion EUR, because of the diameter of the tunnel used and the composition of the dirt under the ground."