All of these other answers primarily address the complexity of creating the wire, but cost really is due to a host of to market forces such as base manufacturing cost (influenced by complexity of design and local labor wages), supply chain cost/availabilty, distribution costs, and economies of scale (how much of each wire type people buy and from where) drive pricing.

Don’t believe me? I encountered a situation at a big box store where a bare copper wire was more expensive than same gauge THHN wire: Is there a reason to prefer bare wire for Grounding Electrode Conductor?

THHN obviously requires far more processing than a bare copper conductor, so explain that one to me as something other than market forces.

All of these other answers primarily address the complexity of creating the wire, but cost really is due to a host of market forces such as base manufacturing cost (influenced by complexity of design and local labor wages), supply chain cost/availabilty, distribution costs, and economies of scale (how much of each wire type people buy and from where).

Don’t believe me? I encountered a situation at a big box store where a bare copper wire was more expensive than same gauge THHN wire: Is there a reason to prefer bare wire for Grounding Electrode Conductor?

THHN obviously requires far more processing than a bare copper conductor, so explain that one to me as something other than market forces.