New York City and Washington, D.C. are 229 miles apart [2]. The Acela has ridership of 3.47 million per year [3], or about 951 on average per day. Acela tickets cost no less than $99 [4]. Plugging in we find the cost per mile can be no more than $7.5 million.
That is 30% the cost per mile of the cheapest tunnel, the Shanghai River Crossing, and 5% the cost per mile of the median tunnel in Figure 5 [5]. On the other hand, the Falcon 9's list price [6] per pound to low-earth orbit is about $1,230. That's approximately 70% to 85% cheaper than the competition [7].
Naturally, you'd charge less than $99 and hope for more than a thousand daily riders. The trick is finding routes for which the price elasticity of demand is high, i.e. where reducing the ticket price by 1/2 generates more than a 2x demand boost.
[1] https://news.ycombinator.com/item?id=15519733
[2] https://www.travelmath.com/drive-distance/from/New+York,+NY/...
[3] https://en.wikipedia.org/wiki/Acela_Express
[4] http://www.nbcwashington.com/news/local/Amtrak-Lowers-Acela-...
[5] https://tunneltalk.com/TunnelTECH-Apr2015-Arup-large-diamete...
[6] http://www.spacex.com/about/capabilities
[7] http://4.bp.blogspot.com/_VyTCyizqrHs/S9IabKEr1nI/AAAAAAAAHb...
> Currently, tunnels are really expensive to dig, with some projects costing as much as $1 billion per mile. In order to make a tunnel network feasible, tunneling costs must be reduced by a factor of more than 10.
So, a factor of 10 is $100m, but we don't know what "or more" means at this point.