I'll have to disagree and perhaps be proven wrong.
This, I think, is an anomoly. Figuring a 5.0 at $1000, mayyyybe $1500 is a bit of a stretch. $2300 for that 4.5 just blows my mind. Maybe it's pressable and the buyer thinks it'll get a grade bump or two.
the 8.0 going for $17000 sets the new benchmark for The really high end books, but not really mid-low grade, I wouldn't think.
A 6.0 should not be a $5000 book, it isn't. Well, it wouldn't be at auction, anyway. Put one up on clink and see what dumdum wants to pay $5000, then you never know.
Interesting with this latest sale, though, to be sure.