The Belgian playmaker was voted the Citizens' Player of the Year in a memorable campaign last time out after scoring 12 goals and claiming 21 assists in all competitions.
De Bruyne's form earned him links to Spanish giants Barcelona and Real Madrid, but the six-year deal signed in January looks like being enough to fend off any interested suitors.
Speaking to Universo Valdano, Guardiola confirmed that it will take a world record sum to prise the 27-year-old away from the Etihad Stadium.
"He's an incredible boy. The clause is €250m. I'm sorry, he's not for sale," Guardiola said. "He's very good. He's a very dynamic player.
"Marcelo Bielsa told me he was his favourite player. He does everything. The other day, I met his parents and you often understand how the children are when you know the parents."
Man City are expected to be without De Bruyne until November due to a knee injury sustained in the first week of the season.