This was the most amazing find of all, and I almost failed to get it, because the one in the store had dead batteries. I had to persist for perhaps ten minutes to get the store to try fresh batteries in it so they'd be willing to sell it to me for $4, instead of trying to sell me the Casio model it was a clone of (the fx-82, I think) for $12.
The top line of the LCD has 12 character cells, with a little space in between them, each 5x7 little square pixels, for a total of 420 little square pixels; and there are 10 seven-segment digits below, each with decimal point, plus two more digits for an exponent, two minus signs, and 15 other miscellaneous indicators, for a total of 111 non-square display elements, 531 display elements in all, more than twice the number of display elements on any other device I examined, and the only one with enough display space for readable words.
It also has a bigger keyboard (60 keys) than any other device I examined, with a better feel than the other similar devices (although not approaching the snappy feel of the FM radio's buttons.) and, perhaps most important, has several times more memory than the other devices — still only around 1024 bits, though. And it's about as slow as the crappy calculator, although it's more evident because it's possible to request lengthier operations; in one test, a sequence of 50 reciprocal operations took it about 20 seconds, or about 400ms per reciprocal.
It's also quite sturdily constructed of hard plastic (probably polystyrene rather than ABS, though), with a slide-on cover to protect the screen (and keyboard) in transit.
The main board of the calculator is small, and predictably, has nearly all the electronics in the whole device in a chip underneath a blob of solder. I'm guessing that the smaller size of this main board compared to the crappy calculator's main board (which included the keyboard) may explain much of why this calculator was cheaper.
There is, however, something that looks like a watch crystal on the the main board; I wonder if replacing it with e.g. a 65.536kHz crystal or a 100kHz crystal would make the calculator run faster.
The keyboard was fabricated in an unusual way. It's a chunk of rubber with conductive bits on top of a printed circuit board, much like the keyboard of the crappy calculator or the 9999 in 1. The PCB is a two-sided PCB with, apparently, through-connected holes; but the substrate is paper-thin, transparent plastic, and the traces are something black. The keyboard sits against a hard backing, giving your fingers something to press against to make contact, but that backing is just injection-molded plastic (polystyrene probably?), rather than being the expensive epoxy-fiberglass composite usually used for PCBs.
I've seen this kind of flexible circuitry before for connecting cables (e.g. the ribbons that join the main board to the display and the keyboard, and usually the ribbons that join a laptop keyboard to its motherboard) but I don't remember having seen it before for a keyboard.
The keyboard also a separate piece of hard plastic for each key, improving its keyboard feel somewhat over the crappy calculator's otherwise similar keyboard. It's still not something I'd enjoy keying Morse Code or playing video games on, but it's noticeably better.
Apparently, someone shipped a machine with an LCD that can display an entire line of text with a retail price of US$4; to boot, it has a 60-key keyboard (the MacBook I'm typing this on has only 80 keys) with a sort of reasonable feel. It's possible that that's some kind of fire-sale price, but the Casio equivalent cost only $12, so I don't think so. The manual claims it to have 17000 hours of battery life on 2 AAs. This all makes me extremely optimistic about my chances of building a device that you can write email on, that can run on AA batteries for a reasonable period of time, with a retail price under $10.