He's trying to hit a price point. We're not completely allergic to the $20M freeway bridge that's about 100ft long, and comes with heavy upkeep costs over the long term. So, $100M per mile with Upkeep Lite™ sounds good when making conversation about the topic. Also the $1B/mile is the cost of the Purple Line extension in Los Angeles. Worst case cost no doubt. A real figure nevertheless.
> So, $100M per mile with Upkeep Lite™ sounds good when making conversation about the topic. Also the $1B/mile is the cost of the Purple Line extension in Los Angeles. Worst case cost no doubt. A real figure nevertheless.
The figure is way out. Crossrail in London has a $20 billion total budget (£15b GBP), with 26 miles of tunnel[1]. But the tunnel is only part of the project, several billion is being spent on new regular track and station rebuilds, so the tunneling figure is really closer to $15 billion.
That's about $570 million per mile, but the Crossrail tunnels are just shy of 50% bigger in diameter than what Musk wants to build (20.5' vs 14'). And as the FAQ says, "reducing the diameter in half reduces tunneling costs by 3-4 times".
LA's tunnel might be costing $1B/mile, but how much of that cost is actual construction versus bureaucracy and mismanagement?
I don't know where this "$1B/mile" just for tunneling figure comes from. That's certainly not the tunneling cost for any current subway extension project in Los Angeles: all-in (i.e. not only tunneling) estimates for the first phase of the Purple Line extension are at $2.8B for 3.9 miles of heavy rail subway; the full Purple Line Extension is estimated at $6.3B for 9 miles of heavy rail subway.
The closest thing is the Regional Connector project, which will be 1.9 miles of light rail and has a budget of $1.8B. But that's the entire project budget. Tunneling costs are only part of it; it's hard to find a finely grained budget, but total construction (including four new underground stations) is budgeted at $1.2B here [1], which is still "just" $600M/mile.
Well, that's some adverse conditions - a silt riverbed behaves quite like water, so you're actually building a submerged part of the city. Look at London's early tunnels and their cost overruns (and the maintenance due to leaks).
