The problem with the economic argument is that you have an underground civilization in the first place. Tunneling through stone is horrendously expensive, especially without explosives. The dwarves for whatever reason have this cave complex that would cost billions to make. There are more reasonable interpretations of dwarf societies, but Zarak is not one of them.
from the blurb on 160:
Quote:
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...dwarves think all surface dwellers* are a little crazy
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* including dwarf surface dwellers. They just plain don't like it.
The distinction between paths and roads was not directly specified in the question, but its a distinction worth making.