They post a WTB on the classifieds or buy an expensive one from a GB or buy it from mkb.com or some other retailer, all of which range from 50-200 dollars.
You also have to check whether the PCB is SMD presoldered, and if you're going through the hassle of making a custom keyboard, you probably want it plate mounted.
I think its more like- 40-50$ PCB, 10$ stabilizers, 50$ switches, 20$ plate, 70$ case (unless you get it for free), 20-30 dollars for cheap keycaps, solder would cost you around 10 bucks for all the switches+soldering equipment.
This would get you something that is on par with like a 100-120 dollar board.
I, for one, see no reason to build a custom keyboard if you aren't going to make it something that significantly differentiates itself from the retail competition.
If you want a custom keyboard for like around 200 there is a korean custom with ergo clears for that price, which i'm surprised no one has bought yet.