Ronaldo moved from Manchester United to Real in 2009 for a then world-record fee of £80m and has since gone on to win the FIFA Ballon d'Or twice.
The 29-year-old has been constantly linked with a return to Old Trafford - despite agreeing a new deal to stay in Madrid until 2018 - but his representative, Jorge Mendes, warned his client would command a huge fee.
When asked what Ronaldo's valuation would be, Mendes told BBC Sport: "Cristano Ronaldo? One billion. His buyout clause one billion, so it is one billion. It is impossible to find someone like him.
"If for any reason the club decide to sell him tomorrow for 300 million, someone will pay.
"He is the best player ever in the world. You can't compare him with anybody else,"
Mendes added that potential suitors for Ronaldo would be wasting their time by making on offer for the prolific former Sporting Lisbon man.
"He will not leave Real Madrid." said Mendes.
