Manchester United will have to pay at least £140m to bring Cristiano Ronaldo back to Old Trafford with Real Madrid reportedly ready to cash in on a move that will also alert Chelsea, Manchester City, Paris Saint-Germain, and other European heavyweights.

It is understood that around 18 months ago Ronaldo and his advisors spoke to the majority of Europe’s blue riband clubs regarding potentially leaving Real. In the event he signed a fresh deal last September with the Spanish giants that made him the world’s highest paid footballer.

Ronaldo earns €21m a year, which is £16.4m or £315,384 a week, with Real paying all of his 52% tax. He costs the Spanish club £34.1m a season. Given his pay equates to £693,844 gross under the UK’s 45% tax bracket for high earners, Ronaldo would have to agree a sizeable pay cut on his basic terms if he were to return to United.

United could further boost his take-home pay by offering him a lucrative image rights deal and other sizeable commercial incentives beyond the normal bonuses awarded for successful on-field performance.

The £140m cost for United – or any prospective suitor – would be the transfer fee of around £60m and his basic wage, should he agree to around £380,000 a-week as a gross figure, which on a four-year contract amounts to £79m.

While Real would want £80m to try to recoup all of the fee paid to United five years ago for Ronaldo, any interested club would hope to agree a smaller sum. Wayne Rooney is United’s highest paid player on £300,000 per week. Given Ronaldo’s ability, which places him in at least a class above Rooney, and his pedigree as a double Fifa world player of the year and Real’s record goalscorer, United would be prepared to break their pay structure to offer him nearer to £400,000 a-week.

Ed Woodward, United’s executive vice-chairman, would be confident of extracting the maximum return if Ronaldo was bought. The forward has the highest following of any athlete on Twitter, greeting the reaching of 30m followers on Wednesday by tweeting: “Wow! Reached the 30 million followers on Twitter, it’s amazing. Thanks everyone.”

Ronaldo’s ability to sell shirts and global appeal would also allow United to ensure the transfer would be a financial – as well as footballing – success.

With the Portuguese turning 30 in February, Real have decided the end of the season will be time to cash in on an asset who will have three years left on his contract by the summer of 2015.

Since signing in 2009, Ronaldo has continually spoken of his affection for Real. Having scoring the 25th hat-trick of his glittering Real career – a club record – in the 5-1 win over Elche, Ronaldo was once more asked about a return to United.

He said: “They are all speculations about my future. My future is Madrid. I’m happy this season is going well and my future I will not speak about, it does not make sense.”

Yet with Real revealing the club is £400m in debt the sale of Ronaldo next close season would make financial sense as he enters his thirties ahead of an inevitable physical decline.

Last Friday Louis van Gaal, the United manager, admitted he would be interested in buying Ronaldo.

Regarding any competition for his signature, it is thought City are not minded to buy Ronaldo, while Chelsea’s José Mourinho described his relationship with the player as non-existent. Bayern Munich may not be able to afford him and PSG may struggle to convince the 29-year-old he should join as French football is not considered on a par with Europe’s best domestic competitions.