I was thinking about the minimum hardware and software needed to get a flexible notetaking device working on an AVR microcontroller or something similar. There are a variety of old Nokia cellphone displays out there that can be accessed via SPI, such as http://articulo.mercadolibre.com.ar/MLA-647101617-lcd-nokia-5110-_JM (84×48 pixels, monochrome, SPI, AR$99). PS/2 keyboards (http://articulo.mercadolibre.com.ar/MLA-624534188-teclado-evertec-kb-12u-bk-ps2-teclas-resistentes-compacto-_JM AR$59, a few blocks from my house) are easy to interface. Supposedly 4GB MicroSD cards are easy to interface via SPI (http://articulo.mercadolibre.com.ar/MLA-648833744-memoria-micro-sd-4gb-micro-sdhc-nueva-sandisk-micro-hc-_JM $49). And the ATMega328 is readily available (http://articulo.mercadolibre.com.ar/MLA-609412290-atmega328-atmega328p-pu-dip28-bootloader-arduino-nubbeo-_JM#D[S:ADV,L:VQCATCORE_LST,V:3] $70).
So the basic hardware costs (+ 99 59 49 70) = AR$277 = (/ 277 15.8) = US$17.53. Plus batteries I guess. And the result has 105 full-sized keys, (* 84 48) = 4032 pixels (8 lines of 24 4×6 characters, say, although originally it was used for 4 lines of 13 characters), and 4 gigabytes of permanent storage — storage you can remove and read and write elsewhere!
I actually already have an old PS/2 keyboard lying around, reducing the cost to AR$218. It weighs about 800 g, and there's a space containing the LEDs with no keys of about 85×45mm where maybe the LCD could be mounted. The Nokia 3110 from 1997, featuring the display I mentioned above, is only 45 mm wide, so the screen must be about 40×25 mm.
But what do you do about the software? At a minimum you need a font, some screen update logic, editor buffer update logic, search, and probably some kind of snapshotting, ideally using the firesystem other machines will expect on the SD card.
The ATMega328 has 32K of Flash and 4K of RAM. You could probably fit all the code necessary for the editor and the LCD, filesystem, and keyboard drivers into that 32K. Ant’s Editor, a simple buffer-gap vi clone written for the IOCCC around 1993, is a bit under 3K compiled to amd64 machine code. But you probably want to do paging from the SD card as much as possible; in particular you don’t want the 4K of RAM to have to contain the whole document you’re editing.
None of these three devices requires extremely tight timing. The LCD is clocked from the microcontroller (its five lines are reset, chip select, clock, data, and “data/command”, of which I think the last three can be multiplexed with other things as long as chip-select is held high), the SD card in SPI mode is also clocked from the microcontroller (ideally at 20 megabits per second, 2.5 megabytes per second, using four lines: clock, data in, data out, and chip select, so again it only needs one dedicated pin), and the keyboard generates its own clock but only clocks at 10–16.7 kHz, which means that each phase of the clock lasts 30–50 μs (according to http://www.computer-engineering.org/ps2protocol/). That means that, although there is a deadline, the deadline is very generous: 10 or 20 microseconds, which is 160 to 320 clock cycles at 16MHz, 200 to 400 at 20MHz, or 80 to 160 at 8MHz if I want to save myself a crystal. And all we have to do at that time is sample the DATA line.
Also, if we miss the deadline, all that happens is that we miss a keystroke.
The whole microcontroller interface needs eight pins: the LCD reset line (maybe optional), two chip select lines for the LCD and SD card, three clock and data lines shared between the LCD and SD card, and two lines for the keyboard. That leaves the other 15 GPIO pins of the ATMega328 free.