The Nasdaq Composite fell 2.9% on Friday, its steepest one-day decline since June, while the S&P 500 lost 1.8%. More than $2 trillion has been wiped off the Nasdaq 100 last month as investors reassess whether the hundreds of billions being committed to artificial intelligence infrastructure can generate returns commensurate with the investment. The concern is not that demand for AI is fading. It is that the cost of building the AI economy — from data centres and chips to power and computing capacity — is rising faster than confidence in the eventual payoff.