Most small run electronics are.
Flash carts will vary by system -- other than needing fast memory there is nothing too drastic about a GBA flash cart, even less for the DS. As chip that acts as a flash cart is not a thing most electronics vendors sell (we saw some oddities with the 3ds and things might change in the future) you need something to handle speaking to the console, and tie together storage reads, save handling and maybe also act to run a menu and have cheats.
Go older systems and the things devs would do to expand the baseline capabilities of a system -- the SNES has any number of fancy chips to help processing (
https://wiki2.org/en/List_of_Super_NES_enhancement_chips https://web.archive.org/web/2008031...ocketheaven.com/SNES_games_with_special_chips ), the NES is not entirely lacking (see mappers (
https://wiki.nesdev.com/w/index.php/List_of_mappers ), the gb/gbc has memory bank controllers with various abilities (
https://bgb.bircd.org/pandocs.htm#memorybankcontrollers ), the megadrive has fewer (it is mainly one driving game with really fancy extras) but if you are going to pretend to speak as a megacd then you are going to need to handle that as well.
Most of those chips are not sold any more, or maybe never were, (there is a reason various N64 and SNES things will have you cannibalise a cart for such things) and even if you could find enough for a run (one chip might be doable, 1000 is a different matter) then by the time you add the 10 or so variations minimum to a board it is full up and you probably also have to add some power switching options there to not cook the console with everything powered at once. To that end you then get to look at chips that are programmable that can either emulate the whole chip or just its responses, and handle all the speaking as a cart things mentioned earlier. Various things available for this but the best of the bunch is the FPGA, these if you program them properly can basically match transistor to transistor the original chip (or anything else you might want) with all the same timings.
Gate count is a bit old fashioned a method to determine the potency of FPGAs but I will go it anyway, you can consider it roughly synonymous with logic elements/cells some vendors will use if you want. You probably want a pretty decent one here, even more so if you are doing more than direct and interpret read requests.
https://www.eevblog.com/2013/07/20/eevblog-496-what-is-an-fpga/ if you want to know more. Programming them is also something of a skill, and while they teach university students every day how to the sorts of things done here is not for the unversed. Programming tools are also something not terribly cheap (
https://www.digikey.co.uk/products/...s-embedded-complex-logic-fpga-cpld/796?k=FPGA ) but maybe you have them at work or school.
https://www.digikey.co.uk/products/...pgas-field-programmable-gate-array/696?k=FPGA
Today we have nice fast SD cards that can store basically an entire ROM set, every ROM hack made and likely will be made in the next 20 years but if you still need some storage of reasonable potency (SD cards might be pretty fast as of a few years back but they are not RAM or NOR fast).
Random search for china pcb fabrication.
Run of 500 100x100mm PCBs, 2 layer (front and back), gold edge connectors on
https://jlcpcb.com/ runs me about $320 (flash carts don't need too many fancy options. 1000 then double that, which naturally has to be paid up front. You might also want a prototyping run (from wires on a bench to home made PCB is one thing, would suck to make 500 of an actual PCB and have to scrap them) or two.
You also have to assemble it as that is just PCBs -- get to put the chips on, and solder it. Doable enough for 10 or so but if you are 500 or 1000 that is a different matter, not to mention one or two will likely fail during assembly and initial testing. This you might do locally or might not.
Oh and time to make a case as well. Probably not going to 3d print that one. Injection moulding is viable at that batch size but still nothing to sneeze at. If you can hot vacuum form something then... it will probably feel a bit cheap but not bad as these things go. Either way more expense. Maybe you can make it fit in an original cart and get people to cannibalise that console's equivalent of barbie horse adventures.
So now you have a massive up front investment to get here.
Website, fees, shipping, things lost in transit, things "lost" in transit, returns of defectives... pretty much want to double the cost of parts and manufacture to stand a chance of not only breaking even but making a little profit enough to do another cart run or simply have some profit (you have likely sunk several hundred hours of work time, which if you are good enough to build a flash cart is something that does not come cheap). Hopefully none of the chips you used have stopped being made else you get to do more fun with PCB design.
Go a video covering other small run electronics costs. You are probably not going to be selling a kit on a popular website like that goes on about, but at the same time you are not going to be selling a kit on a popular website and instead handling your own stuff or doing small batches to fly by night flash cart vendors, plus kit = not assembled board most of the time.
