Actually, if you want a single Calixis-pattern Fury, with no crew or support staff, it could arguably come to a mere -10, or even +0 (-30 for Extremely Rare, +20 for Trivial if you've got no ground crew for it; potentially +30 if it comes with no crew at all (you're only buying 1)). However, I would, and have ruled that a single Common quality Fury, with the crew and support staff, but no pilot, comes to -20 (Extremely Rare -30, Minor +10), as the ground crew and so on makes a single flyer the rough equivalent of a platoon.
In BfK, the smallest unit of Aeronautica you can acquire is a Flight, which is explicitly considered Company-level. A flight is roughly half a squadron, so that -60 is buying you 10 Furies, with pilots, crew, ground crew, munitions, spare parts and the rest of the logistical tail that would be considered organic to such a unit. Well, in theory, you could buy a single plane for -50 (mustn't forget the Size modifier for Acquisitions), which would also include the pilot, ground crew, munitions, spare parts, logistical tail etc., but that would be of limited effectiveness (and still significantly more "expensive" than buying one for a character to fly).
Essentially, that extra -30 comes down to the price you pay for acquiring Mercenaries for a war, as opposed to acquiring them as "household troops" for your own ship. Remember that units Acquired as mercenaries can have a separate set of Characteristics than the crew of your ship, while "household" squadrons default to your ship's Crew Rating. Yes this can mean that it is "cheaper" to hire elite guys for your ship than average guys as a formed unit to fight wars.