The £90m often used by the press is the figure if you were to stay in the Premier League for one year and then get 4 years of parachute payments. So the £90m is over 5 years.
If you weren't relegated, those same 5 years could see the club earn £200m+ in TV money.
None of this takes into account the extra money you could earn through increased sponsorship, advertising, ticket sales, merchandising etc etc whilst in the top flight.