Should you get a 3ds flash cart. The three main reasons to pause would be to consider the 3ds library (I hold the 3ds library to be abysmal with not a lot you can not get anywhere else, and probably in just as good a form there), whether 3ds homebrew is worth it (the GBA and DS did genuinely represent among the leading portable platforms of the day, today we have the likes of android which ruined any chance of that being repeated*) and if there is a 3ds custom firmware or otherwise really cheap flash cart around the corner. Other reasons would be Gateway, the leading option for a custom firmware based cart that runs custom games and what have you,
A custom firmware could be done and may even happen. There is a decided lack of cohesion among the 3ds hacking community though, to say nothing of the anti piracy bent among some of the more prominent/more capable of delivering, so I am not expecting much of note to happen soon. You can get some stuff done but the ease of use is not quite that of a 3ds flash cart.
A cheaper 3ds flash cart... earlier on in the gateway lifetime there was an article on the bill of materials that did the rounds. I have issues with other parts of the article but the BOM was somewhere in the ballpark anyway. Equally the components that make it up could probably be simplified a bit, and some interesting things were being done on the sky3ds when I looked at the PCB there.
There does not seem to be much in the way of competition on the 3ds to drive prices down.
*towards the end of the DS we saw many a homebrew dev migrate to IOS, since then we have seen android take over there. Equally many of my favourite devs from the commercial side of things on the GBA and DS seem to be focusing heavily on andrios.
http://gbatemp.net/threads/links-to-various-gbatemp-features-over-the-years.352851/ showcases many games I like from the GBA and DS, when doing the articles I would often do a "where are they now" and time and time again. I should note though I do not play Nintendo handhelds for Nintendo games (give or take advance wars and starfy, the former of which is rather absent on the 3ds thus far and kind of dead in Japan, the last DS entry never made it out there and only appeared in Japan on the 3ds as a downloadable game, so nobody is expecting all that much). Back on homebrew I imagine it will probably end up with SNES emulator on par with the PSP ones (which is to say playable by most people for most games) but when others are looking beyond the 16 bit era and even contemplating the gamecube then yeah.
This is inaccurate, the original R4 was the first Slot 1 cart available, and initially sold for around $100, It was rapidly cloned and prices dropped fast, though few of the clones duplicated the quality or stability of the original. Before R4 the only way to play pirated games on a DS was by using a Slot 2 (GBA) card like an EZ-Flash, G6 Flash etc. along with a Slot 1 "Passcard", That setup could easily run you $150+ and limited you to using either slow removable memory cards like MMC or Mini SD, or having faster but non-expandable onboard memory (my G6 Flash had just 512MB of onboard storage and cost me $120 (used, with Passcard III).
I fear you are misremembering the way things went. The first DS slot cart was ninjaDS (
https://gbatemp.net/threads/ninja-ds-review.36881/ ), the R4 was not even the first of the big boys to join the game (r4 was oem M3 so I will count them as one of the big boys). It was however the easiest to use and at sold a reasonable price point, if ease of use = downloading updates every other day rather than updating your save list but enough of my griping there, so it came to dominate the market. Equally the G6 was expensive but if you paid that kind of money for a removable memory card you went wrong somewhere (up to and including purchasing some of the earlier neoflash gear).
Also some of the DS flash carts went cheap probably more because people were supporting others which made the older ones without support look bad and vendors took the "something is better than a complete loss" approach to business. For years the R4 was a running joke on lack of support, it was only after woodr4 got released that things swung back around.