A unit cost is the price per unit of measure for a specific construction line item: dollars per square foot of drywall installed, dollars per linear foot of electrical conduit, dollars per plumbing fixture, dollars per door installed. Unit costs combine labor, materials, equipment, and (often) sub markup into a single per-unit figure used to convert takeoff quantities into a priced estimate.
Unit costs come from three sources: published price books (RSMeans is the dominant national reference), historical project data (a contractor's own averages from past jobs), and current sub quotes for specific scope. National price books are calibrated for a national average that does not exist anywhere; using them blindly produces estimates that miss local market reality. The most defensible unit-cost catalog combines published baselines with locally calibrated overrides from your own subs and past projects.