I wish there was more focus on costs. A cheaper project allows more miles to be built with the same money.
Obviously quality is tradeoff you need to be careful not to lose with cheap, but evidence is countries that don't speak English do this at much less cost and if anything better quality.
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.
What benefits? Do you count payment to consultants as a benefit - the consultants would.
If you define benefits as ability for people to get from point a to point b, often, quickly and safely a lot of costs are not needed. Note in particular I artistic stations are not a benifet to me - some would disagree.
We know that by my definition Spain is able to build rail for much less cost.
Note that I picked consultants arbitrarily as something to pick on. Unions have been blamed. EPA has been blamed (I'm not sure what the UK equivalent is). Bribes have been blamed. There is plenty more.
There are always costs to projects (money spent on wages to UK taxpayers, money spent on imports, environmental damage, political damage from nimby campaigns, opportunity costs from people working on this project instead of another one - say crossrail 2)
There are always benefits to projects (money generated from more passengers, benefit of more space on existing lines, movement of freight from road to rail, movement of passengers from air to rail, benefits politically from spending money outside of London)
This is the same no matter what project, or who is funding it. Musk presumably includes "being able to go to Mars in his lifetime" as a benefit of starship, as that's his stated goal.
You could reduce the cost if you said "we don't need to go to mars", but that would remove the main benefit, and Musk isn't going to put as much as he does into it.
Clearly if you can reduce costs without reducing benefits it will increase the viability of a project, but reducing costs on its own does not necessarily increase the viability.
Reducing costs gives two benefits. First the polical opposition has less to complain about. Second, if you reduce costs you can build more for the same money. In transit networks effects are critically important. The more places you can get to (given time, cost, safety, comfort...) the more likely you are to use it. Thus cheap drives better to a large extent.
Obviously quality is tradeoff you need to be careful not to lose with cheap, but evidence is countries that don't speak English do this at much less cost and if anything better quality.