Mendes said the three times Ballon d'Or winner has a one billion pound buyout clause on his contract, and believes the La Liga side will easily get 300 million pounds if they sold him.
However, Mendes reassured Madrid fans that his client will most likely finish his career at Real Madrid.
When asked how much his client is worth Mendes said: "Cristano Ronaldo? One billion. His buyout clause one billion, so it is one billion. If, for any reason, the club decide to sell him tomorrow for 300 million, someone will pay.
"He loves the Manchester United supporters because he was there six years but he will finish his career at Real Madrid, for sure."
In 2009 real Madrid broke the world record transfer fee to sign Ronaldo from Manchester United, paying 80 million pounds for the Portuguese ace.
Mendes was speaking at the launch of his new book. The agent is considered one of the most powerful men in football today, and his client list includes James Rodriguez, David De Gea, Victor Valdes, Jose Mourinho, Radamel Falcao, Angel Di Maria and Diego Costa.