Why is non-metallic cable (NM) cheaper than THHN?
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.