What determines your AC installation cost
System size (tonnage). Phoenix homes typically need more cooling capacity per square foot than milder climates, but oversizing is a common and expensive mistake. An oversized unit short-cycles, wears out faster, and doesn't dehumidify during monsoon season. Ask for a Manual J load calculation.
Efficiency rating. Under current SEER2 standards, the minimum for the Southwest is higher than the national baseline. Stepping up to a high-SEER2 or variable-speed system costs more up front but pays back faster here than almost anywhere in the country because of run hours.
Scope of work. Split system vs. rooftop package unit, whether the air handler and coil are replaced together (they should be), ductwork condition, electrical upgrades, crane fees for rooftop units, and permit costs all move the number.
Rebates and financing. Utility rebates and federal tax credits for qualifying high-efficiency equipment can offset a meaningful part of the price. Ask each installer to itemize what you qualify for.