Get a TEX 60% Alu case, directly from tex, $130-ish at max, include the alu. feet (best 60% case out there in my opinion, you can't get close by custom-cnc'ing)
PCB from winkeyless.kr or GON (You can also get a plate from GON, and stabiliser parts) ($50-150)
Get switches from mechanicalkeyboards.com (<$50 - best switches out there IMO, these days there are bad switches too)
Assemble (maybe lube the switches and stuff)
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I don't think $500/700 is enough for a true custom with a custom case like that, for $500, you can assemble something decent yourself (the parts above will probably become $400+ with itsy bitsy stuff)
Thanks for the suggestion! If $700 isn't enough, roughly how much would I need for a fully custom board?
Since you are vaguely looking for something that doesn't exist, $10.000 would get you what you want
But, as some concrete advice, if you are looking to get a decent keyboard built from existing but high-grade components, I suggest looking for someone to do the buying and building for you
The most important part is the switch lubrication, a person who lubricates switches well should be able to do the remaining things pretty well too, it all boils down to the lubrication
If regular cherry MX clears are 10X for example, a properly lubed cherry MX clear is easily 100X in terms of satisfaction
I suggest looking for someone to build your keyboard, and strucking a parts + labor kind of deal, this way you can have a clear say in the part selection, and pay $100-$500 labor depending on what kind of a person you are dealing with
This is one example of a keyboard laborer:
https://www.reddit.com/r/mechmarket/comments/3gitn2/uspah_keyboard_services_w_paypal/ (sorry off GH :)