^ If no one asks for it and is willing to invest an considerable amount/time to build the first prototypes, then it never will happen.
Also, i don't know what do you mean with affordable. Even 150/200 USD seems enough affordable for such specialized pieces of hardware. You can't ask less for projects that are built in small quantities.

Seems like the kind of thing the Lo-Tech guys could pull off...

1. GBS-8200/8220 could work with CGA/EGA/MDA, provided that you could generate composite sync signal out of discrete horizontal/vertical ones provided by CGA (it requires composite sync signal for anything but VGA horizontal frequency). I got recognizable image (though hardly stable) of XT POSTing with CGA, when I feed composite sync input on GBS-8200 with Composite Video (the RCA jack on CGA board). However, it is hardly practical solution, as memory counting was more or less watchable, but as soon as full-screen editor of Microsoft Cassette Basic was invoked, GBS-8200 lost sync (Generally speaking: the more pixels lit on the screen, the harder chance to get synchronization with (mis)using Composite Video). With GBS-8200 included by default trimpots, one does not even have to use any kind of resistor at the input.

I've also tried some ways of combining both sync signals for GBS-8200 (namely: trying to combine both signals with diodes and probably resistors? I don't remember now if I tried it or not) but failed at it. However, I'm certain that it is possible with some simple 74xx gate or LM1881N - you will find appropriate schematics here: http://info-coach.fr/atari/hardware/video.php … reate-composite
Here is yet another circuit to combine sync signals: https://ianstedman.wordpress.com/tag/gbs-8200/

However, interfacing CGA/EGA/MDA with GBS-8200 needs to take into account that the board works with 3,3 TTL levels, so SYNC signal from LM1881N must be attenuated (not that 5V TTL levels wouldn't work, but this is stressing the onboard DAC very much) - here's some additional info: https://ianstedman.wordpress.com/gbs-82xx-experiments/

2. All that statements that "CGA/EGA/MDA is digital and GBS-8200 takes only analog CGA/EGA/MDA", whose authors imply that it is Extremely Big Obstacle, are apparently made by people that hardly understand how it works. It is technically true, but that issue is trivial to solve. Let me explain:: Fact, that CGA/EGA/MDA is digital only means, that each color (or intensity) could be 1 (5V) or 0 (0V) - pixel lit or pixel darkened. Hence, MDA provides 4 shades of gray as we have two bits of information (video - pixel lit/darkened and intensity - normal/intense), CGA has 16 colors (we have three lines for each color - giving us 2^3 = 8 combinations, intensity bit multiplies it by 2, so each color could have low or high intensity) and likewise EGA has 64 colors (2 lines for each color, so we have 2^6 combinations).

Making such TTL signal VGA-compatible requires just some resistors to bring TTL's 5V to 0.7Vp-p that VGA accepts - of course with different standards we may have to include one bit of intensity or 3 bits of intensity for each color, but making such D/A converter is still a matter of some more resistors. Of course, I'm not talking here about brown color that IBM CGA monitor interpreted differently, if we wanted to be that precise and true to the original, more involved circuity is needed.

Resistor values and method of connecting CGA/EGA/MDA could be devised from here:
http://www.geeksden.info/Sinclair/PC200/CGA2SCART.html

(it is actually cga2scart, but SCART uses 0.7Vp-p too)

There is also another problem with vertical synchronization polarity - it could be either positive or negative. EGA used both in differing modes, CGA was solely positive, MDA - negative, just as VGA. Look here for details. Positive v.sync requires of course inverter of some kind.

3. Hercules and MDA use the same standard and the same monitor. Just wanted to state this clearly and have indexed it by Google.

4. In 80s there were so called 'multi-sync' monitors that could take not only VGA, but also CGA/EGA/MDA-compatible signals. These days are long gone, however it was discovered, that there are at least some LCD monitors that could sync to CGA/EGA/MDA, sometimes contrary to the published specs.

Here is non-exclusive list: http://www.atari-wiki.com/index.php/Flat_Panel_Displays
Note, that this is for Atari ST, but low and mid graphics modes use the same horizontal/vertical refresh so if a LCD supports those, then it should support CGA and EGA (at least to an extent) too.
There are also other such lists on other websites.

Please note that, when testing yet unknown LCD's/CRTs/projectors you might end up with pink-ish picture like one in linked post. This probably means that LCD/CRT/PDP/projector treats input signal as component signal (YUV). This might be easily switchable within OSD.

For the purpose of having CGA/EGA/MDA-compatible LCD, I've bought BenQ BL702A. This is cheap (~$100) 17-inch, 1280x1024 (5:4) LED panel that also got bigger cousins (BL902A/BL912A are said to be similarly forgiving in terms of input signal they accept). There are no official specifications that mentions requirements in terms of refresh rate or vsync polarity. Users manual hints that one could feed this LCD with component YUV signal and LCD should detect this.
Now, unofficial, user-contributed info: It is said to work with all Atari ST resolutions, most - or all - PAL Amiga/CDTV resolutions, so I've did not try these. (I do not own Atari ST; however I own CDTV. Will try this one day).

It displays CGA picture just fine; here you'll find a photo of extremely complicated adapter cable (irony mode off 😉 ) and resultant picture. Note that this does not take intensity bit into account so just 8 colors. If picture quality is not up to a standard, then it is a result of 3 meters long VGA cable and 1-2 meters long RS-232 cable.

I have a DIAMOND FLOWER's VG-7000 graphics card which is nice card in itself, as it could be used on either 16-bit or 8-bit ISA, it is said to be compatible with 8088, has dual monitor output and could be dip-switched to work with monitors representing most legacy standards (CGA, EGA, MDA) - of course with VGA-added benefits like programmable character generator (EPA Polution Preventer logo that is drawn from custom characters is kinda messed though in CGA mode).

I've tried each and every monitor settings of 3 standard ones (MDA, CGA, EGA) and in each mode LCD worked, was able to display high quality image with some minor issues sometimes (for example - CGA does not fill up entire screen - just as on the image that I linked to earlier - I didn't tried to fix that in any way, MDA was shifted vertically - but this could be corrected in OSD). EGA was best - no troubles whatsoever, picture filled entire screen, no need for any adjustments. Of course, it worked with either v.sync polarity.

This is probably the easiest and cheapest way to convert CGA/EGA/RGBi to HDMI
https://youtu.be/A1dDtNDjzGg