I could see a 150 million but not any higher. I think that 300 million number got tossed around erroneously without any citation, then got picked up by mainstream outlets who, as history has shown, know very little about theme parks.
So Everest is $100 mil in 2006, yet somehow Tron and Guardians each cost about as much as Pandora (if $500 mil estimate is true), and together as much as a Galaxy's Edge.
Something doesn't add up here. There's inflation, and then there's Weinmar German chicanery inflation.