Back in the summer of 2009, Real Madrid sent shockwaves through football by paying Manchester United the astronomical sum of £80 million for Cristiano Ronaldo.

The Spanish giants smashed the world transfer record to sign the Portuguese superstar.

However, Ronaldo ended up paying back every penny over the course of his nine-year spell at the Bernabeu.

The five-time Ballon d’Or winner scored a ridiculously impressive 450 goals in 438 games for Los Blancos. These extraordinary goalscoring statistics are unlikely to be repeated.

He also helped the club win a host of major honours, including four UEFA Champions League titles.

Clearly, Ronaldo was worth the money. In fact, it could even be argued that United were left shortchanged by the deal.

Transfer fees have, of course, spiralled out of control in more recent years.

Can you imagine what a 24-year-old Ronaldo would be worth in the current market?

Ten players have been sold for €100 million or more since 2013 - including Ronaldo, who was signed by Juventus in 2018.

But the legendary forward can’t get his head around the fact that certain players in recent years have been sold for such eye-watering fees.

"It's difficult to make calculations in modern football," Ronaldo told Portuguese channel TV1, per Marca.

"[Clubs] gamble a lot on potential and the football industry is different.

Leaving the case of Joao Felix to the side, any player can go for 100 million euros, even without showing anything.

It’s unclear exactly who Ronaldo is referring to here, but let’s look at all the players who have gone for €100 million or more.

1. Neymar | €222 million

2. Kylian Mbappe | €135 million

3. Joao Felix | €126 million

4. Antoine Griezmann | €120 million

5. Phillipe Coutinho | €120 million

6. Ousmane Dembele | €105 million

7. Paul Pogba | €105 million

8. Eden Hazard | €100 million

9. Cristiano Ronaldo | €100 million

10. Gareth Bale | €100 million

With the exception of Felix, who has looked outstanding since joining Atletico Madrid and who Ronaldo put to one side anyway, most of those players at least showed *something* prior to their big-money moves.

Ronaldo also thinks it’s bizarre that centre-backs and goalkeepers are now going for over €70 million.

"There is more money in football,” he added. "A centre back and a goalkeeper can go for 70 or 80 million euros.

I don't agree [with it] but it's the world that we live in and you have to respect it.

Again, let’s take a look at some of the players Ronaldo might be referring to here.

1. Harry Maguire | €87 million

2. Virgil van Dijk | €84.5 million

3. Kepa Arrizabalaga | €80 million

4. Lucas Hernandez | €80 million

5. Matthijs de Ligt | €75 million

It’s hard to disagree with Ronaldo that the current market is out of control, but it’s also hard to see things changing any time soon.