Fergie paid record fees for players from pretty much 1988 onwards. Pallister was a nation record fee for a defender, Keane a British record fee. Fergie bought well, bought garbage, but he/United are a club known for spending more than everyone else coz they had it, they were commercially leagues ahead of everyone else, they are a club known for big/marquee signings, and have been for about 20+ years now.
Ferguson took over in 1986 and from then till ManUtd won first title, Livepool spent more than United, Spurs spent as much and Arsenal just shy of United's spend.
Also in PL era, before Fergie retired, Liverpool,City, Chelsea spent more than United.