The cost to build one mile of 6 lane interstate in an urban area is estimated at 11 million dollars per mile. If you look at the estimated cost for the hyperloop, it seems to come out at around 25 million per mile. This isn't outside the realm of possibility at all, just a little more than twice that of traditional methods.
11 million dollars per mile does not seem accurate. They recently widened the Turnpike in NJ from 6 to 12 lanes over a 35 mile stretch, and it cost $2.3 billion. That's $65 million per mile. And that was considered a successful project.
