The Portugal international was named the best player in the world for the third time in his career at the FIFA Ballon d'Or gala earlier this year and has been in exceptional scoring form for the current Liga leaders so far in the current campaign, netting 36 goals in 31 appearances.
Mendes believes several clubs would be willing to pay £300m for the 29-year-old - a figure over three times that of the current £86m world record fee, which Real paid for Gareth Bale - although he expects the 29-year-old to stay put.
"He's the most expensive player ever in the world, because Ronaldo is the best player ever in the world, the best sportsman ever," Mendes told BBC Radio 4. "You can't compare him to anyone.
"Cristano Ronaldo? $1.5 billion. His buyout clause is $1.5 billion, so it is $1.5 billion. It is impossible to buy someone like him.
"If for any reason the club decide to sell him tomorrow for £300m ($450m), someone will pay. He loves the Man United supporters, he told me, he started there, but he will not leave Real Madrid."
Ronaldo has a contract with Real Madrid until June 2018.
[h/t : foxsports]
