"The machine is a remarkable and unique piece of equipment. Purpose-built for Crossrail by German firm Herrenknecht – one of a handful of TBM manufacturers in the world – it cost around £10m ($15m), weighs close to 1,000 tonnes, has an external diameter of 7.1 metres (23ft) and from cutting-face-to-end stretches 150m (500ft)."
$250 million for a water tunnel that goes 3km. It's simpler than a hyper loop tunnel.
Also I'm not sure Crossrail is a good counter example. That's a 30 billion dollar project to go 73 miles. Saying the TBM cost $20 million is like quoting the cost of a truck without including the price of the driver or diesel fuel.
TBMs are typically built specifically for a job and its ground conditions. Depending on the type of ground, the machine can be a different type of machine that uses different tooling. This will ultimately drive TBM cost. It is possible to reuse machines but this is, again, dependent on geology.
Anyways, the cost of a TBM is generally not the most expensive part of a tunneling operation.
Labour costs. TBMs are amazing but they are slow and require a whole lot of infrastructure. It's not just a case of sticking them in the ground, turning them on and waiting for a nice tunnel to materialize. For instance, what do you with the excavated dirt? To give you an indication of the sheer cost here, London's crossrail project runs at approximately £4m/day - https://en.wikipedia.org/wiki/Crossrail
