What will tend to put your guesstimator off when it comes to SCSI, is that the SCSI drives you think are equivalent to IDE drives in size and speed, were probably 2 years older than those. Then they probably only had a tenth the consumer market penetration. When they were used en-masse in servers and power user workstations, they were usually scrapped en-masse when retired. Then also when they were used, they were USED, beat on daily, so attrition from wear is much higher. Quite often in Novell, NT server deployments, offices and colleges etc, the few hundred desktops would almost not touch their own IDE drives they'd boot off the server, get applications off the server, save work to the server, so their drives got very light use, while the SCSI of the same age did it all. So those were wore out but maybe some of those IDE drives are still kicking.
With Macs and Amigas, it gets to be that same "known to work" thing that tends to happen to pre 1990ish IDE, the BIOS or whatever likes this particular drive, works with this particular drive, so those SCSI drives and the type 17, 26 or whatever BIOS favored IDE drives go for larger coin. Modern example is what drives an Xbox 360 likes.
Many also were not cheap in the first place, like Obsidian graphics vs run of the mill Voodoos, I've got one hulking monster of a 1GB drive that if I was asking a tenth of release list price for it, it would still be several hundreds. That would be because at the time it was made 1GB was unfathomably brain exploding large, I think premium systems were coming with 120MB tops.
Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.