Quite a few comments question the cost of digging -- which is understandable. But the context to remember is that these are huge infrastructure investments that will return revenue from that investment over 20-100 years with little maintenance costs. The excavated shaft is like a factory not a product. The product is each cycle of energy generation giving you a profit day after day. The shaft is a one time investment that will yield that product forever.

There's lots of research showing how the costs work out (see my other comments here)