Maybe I’m missing something but last time I checked there was no ergonomic keyboard cheaper than 300 USD. Is that really the case? Do they become (super) less expensive when DIY?
It depends on what kind of ergonomic keyboard you want I suppose. You could get an assembled 36-key GergoPlex for $160[0]. And I do think it's cheap to build a dactyl-manuform yourself if you have access to a 3D-printer.
The most expensive single part of the custom mechanical keyboards is the case. If you want a high quality aluminum case, those usually double or triple the price of the keyboard. Off-the-shelf options that use plastic cases are probably your best option to minimize costs. Kinesis makes a few models that are pretty nice. Ultimate Hacking Keyboard is just under $300 (w/out shipping). X-Bows is around $200.
DIY boards with sandwich case is the cheapest option, but they don't look or feel anywhere near as robust as boards with full case design. You also need to factor in cost of tools to assemble one, and switches and keycaps, so it actually ends up being pretty close to the examples above.
I'm quite fond of the kits sold by https://www.littlekeyboards.com/ They have a good selection of some of the more popular split ergonomic keyboard designs mostly from Japanese keyboard designers with different kits, incl. different case options. I believe they will also assemble the keyboards for you at extra cost.
With all the options (larger cable in between the halves, risers, etc.) it's closer to $200 than the $99 base price but still a tremendous value.
An added benefit over most other super ergonomic keyboards is that the design is close enough to standard QWERTY that it's pretty trivial to get going. I've a few friends who have really struggled with the more aggressively altered layouts of some of the other designs.
I DIYed an ErgoDash for about €150, mostly from keycapsss.com. That meant buying a PCB, case, parts kit, Arduinos etc from them, and key switches and cheap keycaps from China.
You can further reduce the cost by ordering the PCB yourself from China, and some components from electronics suppliers, or laser cutting a case yourself at a maker space (or doing without).
It's also possible to do without a PCB, and handwire the whole thing. This is common where there's little alternative, like on the Dactyl or Dactyl Manuform 3D keyboards.
[0]: https://www.gboards.ca/product/gergoplex