Neuralink cofounder Elon Musk.Fabrice COFFRINI / AFP via Getty ImagesInvestors are valuing the brain chip startup Neuralink at $42 billion in secondary markets.Neuralink was last valued at $9 billion in a funding round over a year ago.Since then, Musk has led a blockbuster SpaceX IPO, which is now worth $1.7 trillion.Demand for a piece of anything related to Elon Musk is booming.The near-trillionaire's brain chip startup, Neuralink, is being val…