The answer
AC replacement costs $4,000–$9,000; Gas Furnace costs $3,500–$7,500; Full split system/heat pump costs $7,500–$16,000+. DOE minimums are 13.4 SEER2 (North) and 14.3 SEER2 (South). The 15.2 to 17 SEER2 tier offers the best ROI: 20%-35% electricity savings over older units, qualifies for ENERGY STAR and $2,000 IRA tax credits, without the steep price premiums of 20+ SEER2 systems.
Sources
This answer is drawn from the standards and guidance below. It is general information about how these systems work — not a diagnosis of your property.