If you have the kind of nice geotechnical conditions that permit the use of tunnel boring machines then tunneling isn't actually very expensive. What's killer is the cost of subway stations, which need to be dug out manually.
I thought cost-savings from a system that used narrower bores was the key to Musk's whole strategy? I thought that was the whole thing - by designing smaller hardware he could make underground transport affordable?
To clarify, I was talking about traditional subway construction and why changes in boring costs would not change that.