From the thread on 401 W. Lane St. that somehow devolved into a conversation about rail construction:
It’s hard to say for different digging methods beyond the contexts of individual projects, but one of the researchers behind the Transit Costs Project noted in his professional blog that cut-and-cover construction is like 50% ($120M) cheaper than boring tunnels, per mile.
Another example: CalTrain, the commuter rail line connecting San Francisco to the rest of Silicon Valley, compared the costs of electrifying their tracks and grade-separating them. Compared to at-grade construction, the average aerial viaduct in its right-of-ways were 3x as expensive, whereas cut-and-cover and boring would’ve been 5x and 7x, respectively. But again: take that with a grain of salt because we’re talking about one of the most expensive real estate markets on the planet.
This study from the influential Eno Center for Transportation also found that the cost of tunneling for rail projects is artificially higher in the US than the rest of the world. This matches up with what tunneling industry experts have noticed have found that the construction itself is only a part of why tunneling is expensive: all of those reports blame sloppy regulations (e.g. NEPA being warped to promote NIMBY interests) and decision-makers being allergic to temporary disruptions (to people’s daily lives as well as to traffic).