Scotchegg has already touched on this but.....
Norwich actually still get parachute payments (it doesn't necessarily feel like it, but they were only relegated from the Prem three years ago).
Derby make a big loss most every year, and currently Steve Gibson is trying to get the EFL to investigate them for ignoring sustainability rules.
Leeds' owner has apparently been pumping 15m of his own money into the club on an annual basis to keep them competitive.
Championship clubs spend silly money on wages nowadays. There's only one team, really, that's managed to defy the financial odds and compete at the top of the Championship without losing a boatload - and that's Sheffield United. When you look closely at the details, the job Chippy has done there really is extraordinary.
Unless Sunderland can find a similar over-performing manager, then it will be a struggle financially for them back in the Championship. Unless they get there immediately whilst they're still receiving the Premier League's money.