When a gas furnace is replaced, the cabinet is the visible part of the job. The ducts that carry warm air through the house were installed earlier and often outlast the furnace. They decide how well the new unit can perform, so their condition and size belong in the cost picture from the start.
Duct size sets the limit
If the trunk or branch runs are small for the new airflow, the unit has to push air through a passage it cannot fill well. Ask the installer whether a duct check is included, what it measures, and whether any duct size change appears in the written scope. A change you did not ask for is a cost driver, and you should see it on paper.
Leaks and sealing
Leaks at joints and at the plenum send conditioned air into attics, crawl spaces, or wall cavities. Sealing is often listed as its own line. Ask whether sealing is included, whether it is optional, and whether the sealed joints are checked again after the work.
Return air and the filter rack
A furnace needs a return path that moves air back to the unit. A small return, or a filter rack too tight for the new cabinet, can force a change nobody planned. Check the filter size on the old unit, then ask whether the new unit takes the same size or needs a rack or cabinet change.
Compare the drivers, not only the totals
Two proposals for the same furnace can differ because one assumes the ducts are sound and the other includes duct work. Ask each installer to name the drivers they priced: duct size changes, sealing, return changes, and any optional items. Set the lists side by side and read them line by line. A lower total means little until you know which items are missing from it.
For a planning frame that does not depend on any one bid, the gas furnace service life and when to plan page on this site gives the published figures.