AC and furnace replacement cost cannot be reduced to one reliable national price. The useful comparison is the written scope: diagnostic work, exact equipment or component identity, access, compatible parts, labor, refrigerant handling when applicable, electrical or duct corrections, permits, commissioning, warranty terms and disposal. A low headline figure and a complete installed scope are not the same product.
Cooling and heating design conditions, fuel and electricity markets, inspection requirements, equipment availability, and building access can change the combined scope. Collect the service history and present condition of both systems, their shared blower and ducts, thermostat, fuel and electrical connections, venting, and comfort complaints. Coordinated replacement may address equipment matching and shared installation work, but it should not be assumed necessary merely because one side has failed. A contractor can assess loads, system matching, airflow, controls, fuel and vent requirements, electrical service, drainage, and which existing components remain suitable. Ask each option to state whether both systems are included, what shared work is required, which components remain, how commissioning is performed, and how warranties interact.