It is possible to build a mechanical keyboard, and a sweet one at that, on the cheap. Relatively speaking, of course.
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PCB & Plates: Fourier 40% Staggered Split
Switches: KBD Fans
Aliexpress for everything else:
Keycaps: YMD Tech DSA Keycaps
Pro Micros: TENSTAR ROBOT Pro Micro ATmega32U4 5V/16MHz
TRRS Cable: 2.5Ft 75cm 1/8"3.5mm Metal Coiled Male to Male 4 Pole TRRS