Cristiano Ronaldo will cost a staggering £785million should a club wish to buy him from Real Madrid.

That's the buy-out clause placed in the 29-year-old superstar's contract and the astronomical price his agent, Jorge Mendes, insists he is worth.

The one billion euro tag underlines why Manchester United and other suitors will find it near impossible to prise him away from the Santiago Bernabeu.

Cristiano Ronaldo's buy-out clause is a whopping £785million, a price his manager says he is worth

Ronaldo's manager Jorge Mendes says the figure means it will be impossible to take him from the Bernabeu

Ronaldo in action for Portugal against France on Saturday night in a friendly international

Sportsmail reported last month how, despite their affection for the player, United were not willing to break the bank for such a deal and Mendes said: 'I no longer get calls from anyone asking about Cristiano because they know that it is impossible to get him out of Madrid, so it's not worth them calling to ask for it.

'The price is that stated in his release clause – €1bn (£785m).

'United are in his heart and he is very respectful of people who show him affection but he is in Madrid to the end.'

The buy-out clause dwarfs that of Barcelona's Lionel Messi which is set at £331million and Mendes says the valuation is justified due to Ronaldo's standing in the game.

'In addition to being the best footballer in history, he is the greatest athlete of all time. You cannot compare him with anyone,' Mendes told AS.

'He is the best player in the biggest sport in the world, in which there is more competition than in any other. For that reason, he is the best athlete in the world. There will be no-one like him for the next 500 years.'

Ronaldo, celebrating a Champions League final penalty, is the 'greatest athlete in the world', says his manager

Ronaldo's rival for world's best player, Lionel Messi (here for Argentina against Brazil in China on Saturday), has a buy-out clause at Barcelona of £331million

Ronaldo got a warm welcome from fans when at Old Trafford in March 2013, but won't go back there

Mendes also believes the Portugal captain will be able to break his own personal record of 60 goals in a single season provided he can stay fit in 2014-15.

'I'm sure that he will beat his personal record if he does not have any physical problems,' he added. 'He is phenomenal.

For Cristiano, nothing is impossible. He is going to continue evolving and improving until he is 40. When he finishes it will be impossible for anyone to beat his records.

'Notice how Cristiano is a winger, he's not a striker, and his numbers are outrageous. He plays very far from goal and has to make significant efforts to arrive into the box. For that, he deserves even more credit for what he achieves.'

And Ronaldo's next step?

'He always wants more and more. His dreams are short term and his dream is to win another Champions League.'

Madrid, meanwhile, are continuing to monitor David de Gea's impressive form at United. There has been increasing clamour in Spain for 23-year-old de Gea to replace the under-fire Iker Casillas in the national side and Madrid are considering a similar move at club level.

Real Madrid is looking at Manchester United's in-form Spanish keeper David de Gea

De Gea (centre) is seen as a possible replacement for Spain team-mate Iker Casillas (bottom)

United though are planning discussions over an improved contract with their goalkeeper and hope to ratify a new long-term deal soon. De Gea, who was player of the year for the club last season, signed for £17.8m in June 2011 and his deal runs out in 2016. Although initial conversations took place last year, talks had been put on hold as United strengthened their squad in other areas.

Louis van Gaal still wants another goalkeeper to provide competition for De Gea with Anders Lindegaard looking for a new club.

Meanwhile, Ryan Giggs will be given the Legend of Football Award at the HMV Football Extravaganza on Tuesday at the London Hilton Hotel on Park Lane.