It's actually more like billions of $ per mile. How anyone looks at a figure like '1 billion per single mile of tunnel' and thinks it's feasible to run utopian pie in the sky experiments like hyperloop across hundreds of miles is beyond me.
HS2, under construction in the UK, is 155 miles long, with a total budget of £55bn but currently expected to cost well over £80bn - some estimates have it as north of £100bn. $1bn per mile is in the same ballpark.
In the UK everything costs twice as much as originally estimated and takes twice as much time. I hope I will live to experience the 330km/h (operating speed of UK's HS2)