One thing that isn't as obvious at first is how tunneling traffic under a city can greatly increase the walkability of the city. Look at Boston after the Big Dig. Neighborhoods that were previously cut in half are coming back together. Downtown is seen as a desirable place to be. It's a big QoL improvement for a city.
I'm not sure if he will be able to bring the costs of tunnel boring down as much as you would like, but I'll hold out hope. At the very least he does have a flair for picking company names, "The Boring Company".