If you are prepared to scrounge parts from used market, you could potentially get all your keycap, switch and stabilizer from an old G80 or whatever under $30, $20-30 for a plastic poker/pure case, and whatever the final cost of the pcb. I guess I would estimate $80-90 for such a build to be conservative and then up to whatever $400+ depending on case and keycap options and so on.