Square footage is the starting point, but it is rarely the deciding factor. The biggest variable is what has to happen below the surface. A driveway installed over an existing stable base costs much less than one that needs full excavation, new processed stone, and drainage correction.
Site access also matters. A driveway that paving equipment can reach easily is more efficient to build than one with tight access, steep grades, or obstacles. Asphalt thickness, the number of courses, and the condition of the subgrade all factor in as well.