Well, nice try, but I wouldn't take these theories to the bank just yet.
First the 512Ke is not all that common. It replaced the low-end line about 4 months after the identical 512/800 did in Europe. Chances are it lasted little more than a year (we don't have an exact date). Since the SE bumped the Plus down to entry level, the 512Ke (which probably wasn't all that popular to begin with) probably got relegated to education only for a while longer. It also got a facelift somewhere around the beginning of '87, becoming Platinum colored and changed to the same front bezel as the Plus, which is probably the rarest of all Macs, but also looks pretty much like a Plus from the front. It also was sold as a Macintosh ED and some even branded as such, so that is an even rarer oddity, but you don't see people getting all worked up over them.
So why aren't they highly prized? That's easy. Because they aren't the first. People wouldn't be clamoring for a 128K if it weren't the first of something. I mean, come on, 128K RAM, hard to create startup disks for it, difficult to use, hard to find early enough software for it ... who would want such a thing? And I love the confusion that crops up on the internet all the time with people thinking that since the 128K is the highly collectable Mac, they think the one with the red 128K label is the first one, cause it says it is what they are looking for! Collectors can be such dolts and sheep. I am always amazed at how they seemingly jump off the deep end without doing any research at all.
The 512K is a cheaper substitute – it was the SECOND Mac ever made! So it still has some cache, yet doesn't cost as much as a 128K (because it wasn't the first). But collectors that pay thousands aren't interested in the poor 512K, which is actually a much more valuable computer due to its usability. And there are far many more 512Ks than 128Ks because it was the first Mac that actually worked the way people wanted it to, including the ability to use a hard disk drive. And sorry JDW, as much as I share your enthusiasm for the 64K ROM chips, most people don't have a clue what's supposed to be on the inside of an original Mac.
But the 512Ke isn't very exotic. It has the heart of a Mac Plus (which are a dime a dozen), it was the 4th Mac ever made, and has none of the "rare" parts which can be used to restore a 128K or 512K. There's just nothing sexy about it in the rush for collectors to get their hands on the first Mac, or as a runner up prize, the second. The thing that kills a 512K for the seller or buyer is that pesky serial number. It's a dead give-away. Or the model number. And of course anybody who actually wants to buy a 68000 Mac to use it, is going to go after a Plus or an SE.
FInally, I question Quadraman's sample. I know for a fact the 128K is not rare at all having built up a database of those offered on eBay for over a year. I agree with JDW that the word is used by sellers on eBay to sensationalize their listing and get novice buyers to forget themselves in the heat of the bidding, buyers who clearly don't do a lick of research. But, I see 512Kes all the time on eBay. In fact many 512Ks are Kes, as many have had the 800K drive upgrade and 128K ROM upgrade. Or they're just listed as 512Ks without even realizing it's actually a 512Ke, and often have to ferret that out by asking the seller questions. I saw one listing where the 512Ke had been "restored" to a 512K, but it had a late '86 serial number! Did the seller just not realize, or was he trying to capitalize on the greater perceived value of a 512K but putting a spare 400K drive into it? I seriously question if he replaced the logicboard or the ROMs, but I could never get that question answered because it was conveniently boxed up already.