Generally, the percentage of price drop is based upon the seller's profit; i.e., the "worth" of the car - its intrinsic "value" - comes from the price of the nuts and bolts and panels and wires, plus the cost of the labor to put it all together. That loss is equal to the profit and not much more.
With cars, it seems that this concept has finally soaked into the minds of the buying public. Not so for musical instruments, at least not for new ones.
After that profit is gone, the rest of the transactions are what seller wants and what buyer wants, reconciled as the final purchase price. Emotion shouldn't enter into it, but (as you have noted) it often does.
Some of the prices on eBay are way off of the charts. I love Series 9 clarinets, and have bought a little more than my share over the years. But, an A clarinet priced at $3,000 plus isn't worth it to me, or to most players who don't play an A horn each and every day of the year.
But, that is the great thing about auctions - they have a wonderful habit of shaking out all of the posturing and the like, leaving you with the real (and sometimes horrible) truth.
My mother sort of had this experience when her "estate sale" was held a few years back. She had an extensive collection of Hummel figures, accumulated over a long life. While we kept back a few of of these with some sentimental value, the bulk of them had to go.
In a two day sale, thousands of pounds of stuff, ranging from a ski boat and massive amounts of furniture on down to the contents of kitchen drawers was put on sale. The gal who ran the sale did a good job of pricing the majority of the stuff. However, she put the prices on the Hummels at what seemed to us, raised with the "value" of these things, at a very low level.
I brought this up, and she replied that it didn't really matter how much she put on them, they wouldn't sell. And, sure as a gun, she was right. even though prominently on display in the great room, not one of the total went for the price posted. In the end, they were "gifted" to her salvage crew as part of the "pickings" they got for cleaning the remains of the sale from the house.
(Other things, to which we assigned far less value, were priced "high" by the sale lady, and went for ever penny that was asked of folks. She knew her craft, I have to hand that to her.)