The cost is proportional to what kind of environment you're operating in. If it's ideal terrain it'll be cheap. If it's difficult, ever changing conditions with

The cost is proportional to what kind of environment you're operating in. If it's ideal terrain it'll be cheap. If it's difficult, ever changing conditions with a high risk of subsidence damage to the structures above, get ready to pay way, way more. See the Seattle "Big Dig" project for an example of how sideways things can go if you hit an unexpected snag.