One is the frequency of service (HS2 Phase 1 is built for 18tph in each direction, and post Phase-2 higher passenger numbers than almost any other high-speed line),
Another is the cost of land (especially at the London end), as unlike many other international projects a dedicated line is being constructed all the way into the terminal (in part because existing lines don't have the capacity),
And finally the increased amount running in tunnel through the Chilterns versus the original proposal (as a result of legal challenges, themselves costly).
The price difference is much less extreme if you compare like-for-like (i.e., compare urban construction with other urban construction, rural with rural), for example.
The cost per mile also includes major station works at Euston and Manchester, more work at places like Crewe and I assume on the east side of the pennines, and new stations at Old Oak Common, Birmingham Airport, Birmingham Curzon Street, Manchester Airport and Toton, as well as including new rolling stock
It is generally believed (though it might not be true) that cost of "luxury" stations is a large portion of the costs. Whatever, if we could build cheaper we could afford to build more.
One is the frequency of service (HS2 Phase 1 is built for 18tph in each direction, and post Phase-2 higher passenger numbers than almost any other high-speed line),
Another is the cost of land (especially at the London end), as unlike many other international projects a dedicated line is being constructed all the way into the terminal (in part because existing lines don't have the capacity),
And finally the increased amount running in tunnel through the Chilterns versus the original proposal (as a result of legal challenges, themselves costly).
The price difference is much less extreme if you compare like-for-like (i.e., compare urban construction with other urban construction, rural with rural), for example.