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Building a keyboard does generally cost more, as you only buy enough components for one keyboard and don't get the volume discounts that the major manufacturers would get. For example switches could cost, say, 30c each, and you'd need around 100 (or would probably buy a pack of 100) which would be $30. A large keyboard manufacturer would probably buy 100,000 switches at once, and get them for 5c each or something.
Then you'd need diodes, resistors, PCB, case, controller, perhaps a little wiring, some plugs/sockets, plus the equipment to build it, plus the time to build it.
Assuming nothing goes wrong, you'd end up with a keyboard you could say you build yourself.
Or spend $80+ to buy a ready-made keyboard at a store. Easier, usually clear channels to return it if defective, you can start using it straight away
)
But if you would like to build a keyboard, have a look through
making stuff together for some inspirations from what other have done.