The pair were signed by Chelsea in 2011.
"We raised the bar on transfer sums at that time. When I arrived, a million euros was a lot of money," said Degraen to Het Laatste Nieuws.
"Chelsea's first bid for De Bruyne and Courtois was €3.5m. For both of them."
He also revealed: "Aston Villa would provide six million for Benteke. Don't hesitate, management said. Negotiations continued day and night, leaving 10 million. I miss the excitement of negotiations."
De Bruyne and Courtois would eventually move to Chelsea for a combined £14m.