The MCU in each module is probably one of the cheapest parts in that build $0.2-0.25 a piece. It's the mechanical parts which are expensive. The switches+keycaps which you need anyway depending on how nice you want them to be can cost $0.4-1.5 per key. The most expensive part is probably the pogo pins $0.5/per pair x 8 = $4 and magents.