If there's ductwork, it's something being installed into the building that has to pass inspections. It's not just one or two holes in the walls that need to be sealed.

But the article said he used existing ductwork, and yet it ended up at $14k.

The new heat pump system cost roughly $14,500 and provides heating and cooling via ductwork that was already in place.

No one in their right mind runs ductwork between the interior and exterior of the building in residential installations. There’s power, possible control wiring, and two or (in exotic installations) three refrigerant pipes.

(In a more enlightened world, I think that a system with the refrigerant fully contained within the outdoor unit that uses plain water as the heat exchange fluid could make a lot of sense, and this would not change the pipe situation.)