Site conditions that move the number
A finished basement mechanical closet, a rooftop or attic unit, occupied living space during the swap, and protecting flooring or landscaping during access can each add real labor time.
Putting the scope in writing
The line between repair and replacement, the system's rated capacity, material compatibility with what's already installed, and how testing and disposal are handled should be written down, not assumed.
Permits, inspections, and other trades
Some system-replacement projects require a mechanical permit, inspection, electrical-panel review, or another specialty trade — confirm which apply with the provider and the city's Office of Building.
Planning context only, not a quote
None of the figures here are an offer to perform work. Insist on a written, property-specific scope and price before agreeing to anything.