It's the old asking price / selling price game. When asking prices get ridiculous, less ridiculous prices seem reasonable.
Selling prices might seem more reasonable if those high money amps were no BS fully serviced. That means no booby trapped crap that's going to blow up somewhere down the road.
It used to be the old black panel Bandmaster was the red headed stepchild of Fender amplifiers. People bought them because they were cheap. Loads of them got hacked up and modified.
For my money I'd track down an AB763 '68 silverface. I'd stuff it in a combo with a couple Eminence Copperheads, sort of a Poor Man's Vibrolux.
I think the prices people are getting for old Fenders is based in large part on what people are paying for similar quality hand wired clones.
From where I'm standin' I barely see any relationship at all.
It takes a lot more work to wire a Twin Reverb compared to a Princeton Reverb. Original Twins are twice as much amp for half the money, sometimes 1/3 the money.
Remember, kids...
fully serviced = more dough. I see amps every day that someone "looked at and gave a clean bill of health". That's often all they did, they looked at it.