The Brazilian, who has had an inconsistent period at the Bernabeu due to injury, has been linked with a move back to former club AC Milan but Mourinho has insisted that the 30-year-old will not be allowed to leave on the cheap.
"If Kaka stays I would be happy and I am happy if he wants to go," Mourinho told AS. "In football it is normal that the players play where they want.
"He is a very good footballer and prepared to be at his maximum level. If he decides to stay, then perfect. If he goes, that's perfect too but we are not going to give him away for free. Those that want him have to pay for him."
Meanwhile, the former Chelsea boss has revealed that he would one day like to coach in the MLS.