That rumor was debunked immediately but is still somehow making waves thanks to PS fans...you'd be surprised, almost all first party games cost that much now.
wasn't it rumoured that Halo was at $500M? you better believe Starfield will be up there.
Only games that I can imagine costing 300+ million (and that's development plus marketing) are Call of Duty games and Rockstar games, Cyberpunk 2077 and Star Citizen