seat belts in an Arrow many years ago. Was quoted about $350.00 for each inertial. Think engineer got them from Chevrolet as the mechanism was identical and only $80.00 They worked perfectly.
Well, maybe they worked, but did they meet the TSO requirements? Labeling requirements? Flammability requirements? So maybe they were not entirely compliant with the aircraft design, and maybe no one would notice until the AAIB go looking during the investigation.
Many aircraft parts I can think of are automotive parts (though not seatbelts nor shimmy dampers). The automotive parts were demonstrated to meet the requirements, and specified in the type design for the aircraft, occasionally as the automotive part number (hose clamps, for example). But other parts may have subtle differences, or changes to make them compliant (like fire retardant applied to aviation seatbelt webbing), which the type design specifies, but is not apparent when you are contemplating swapping out for the cheaper one.
It is funny watching a money saver replace a genuine Cessna battery contactor with the apparent automotive equivalent. Very expensive smoke is the result. Cessna does not tell you that theirs are wired differently inside!
It is well known that several aircraft manufacturers are now less than enthusiastic about their older legacy aircraft flying, so they are not providing attractive pricing any more. To fly (maintain) a legacy airplane, you gotta want it!