[gorbachev]

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.