[YeahSureWhyNot]

A relatively small construction/infrastructure project (920M) will not maintain the same cost efficiency when the project is 80 times larger.

[ummonk]

Right. It will be cheaper per mile with a larger project.

[dkislyuk]

SF Central subway cuts through one of the densest parts of SF (including crossing a BART line and an existing MUNI line), which substantially increases costs. Further away from the downtown core, costs per mile should be lower.

[Ericson2314]

If anything it would be more efficient.