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
Anyways, the cost of a TBM is generally not the most expensive part of a tunneling operation.