The summary: the hard part of tunnels and people was what to do with exhaust gases. That's solved with electric cars. The tunnels will probably pay for themselves or at least break even.
In 5-10 years, its likely that most major freeway projects will have a very difficult time getting funding as tunnels will be cheaper to build and much much much cheaper to maintain than roadways. And existing investments in tollroads or bonds for roads will be selling for pennies on the dollar.
The cost per mile may even get as low as $100k/mile. At that price point, individuals could be drilling their own private tunnels.
As a comparison against tunnel prices now, when Google Fiber tried to lay fiber optic cable in San Francisco a few years ago using shallow trenches and not full tunnels, the full cost was estimated to be in the $100's of millions for the backbone and low billions including all house hookups. https://sfbos.org/sites/default/files/FileCenter/Documents/5...
And San Francisco is a tiny city: 7 miles by 7 miles. Once the Boring Company gets prices down to $1mil/mile, you could make the same Fiber backbone with 3 tunnels N-S and 3 tunnels E-W for 6x7 = $42million - a little more than 1/10th the quoted cost.
More comparisons: the Alaska Way Viaduct project (SR 99) in Washington State cost >$3billion and was only 2 miles long and took 12 years. The cost was $1bil/mile - so Boring Company will be 1000 times cheaper. https://www.wsdot.wa.gov/projects/viaduct/budget
The San Francisco BART extension cost $3bil/mile!! https://sf.curbed.com/2018/6/18/17464616/bay-area-subway-tra...