The CIES Football Observatory research team has developed a unique approach to estimate the transfer value of players on a scientific basis. The model includes multiple variables on player performance (minutes, goals, etc.) and characteristics (age, contract, etc.), as well as data on employer clubs and potential recruiting ones.

In June they revealed Neymar (€210.7 million) was the world's most expensive player and have also covered the most expensive U21 players.

So what would it cost to sign some of the world's top talent this summer? Here is a breakdown of some of the most sought-after players, according to CIES' numbers.

Antoine Griezmann, 26, Atletico Madrid: €150.3m

A new contract and a transfer ban imposed upon Atletico means Griezmann will likely stay put this summer. But a monster offer could still tempt the club to sell.

Cristiano Ronaldo, 32, Real Madrid: €112.4m

Age is the main reason why Ronaldo is not sitting at the top of the tree. He may have signed a new long-term contract but now that he seemingly wants to leave Spain, Real Madrid would still ask over €100m.

Romelu Lukaku, 24, Everton: €105.4m

The Belgium striker is one of the hottest strikers around and Everton know that. He has two years left on his contract and wants to play Champions League football, but has age on his side.

Alexis Sanchez, 28, Arsenal: €95.9m

Arsenal are panicking over Sanchez as he will be a free agent in 2018 unless he signs a new deal. His transfer value is dropping with every passing week and Man City and Bayern will be waiting to snap him up.

Kylian Mbappe, 18, Monaco: €92.6m

The Monaco phenomenon is the world's most sought-after player. Mbappe may sign a new contract and choose to stay in France, but anyone who wants him will have to pay a possible world-record fee. His deal expires in two years, but the hype around him has increased his value.

Ousmane Dembele, 20, Dortmund: €87.1m

The 20-year-old winger is reportedly attracting the interest of Barcelona and Manchester United but has a contract until 2021, meaning his transfer value is strong.

Diego Costa, 28, Chelsea: €83.4m

Atletico Madrid may wish they could sign Costa now, but have to wait six months. Chinese Super League clubs are sniffing around but his value will drop as clubs understand that Chelsea want to move him on.

Pierre-Emerick Aubameyang, 27, Dortmund: €81.5m

Aubameyang has made no secret of his desire to move away from Dortmund, but has a deal until 2020. Man City, Liverpool and PSG are all reportedly interested yet over €80m is a lot of money to spend.

Andrea Belotti, 23, Torino: €71.9m

Torino have their own idea what their star striker is worth, over €100m, but his relative lack of big-game experience means a more modest price is what he is worth. With a contract until 2021, though, it is the club that holds the cards.

Jan Oblak, 24, Atletico Madrid: €65.1m

The world's most in-demand goalkeeper has a contract until 2021 and PSG are most interested. Man United may come in for him if they lose David De Gea -- who is worth €45.3m according to CIES -- but Atletico would be loathe to sell while under a transfer ban.

Riyad Mahrez, 26, Leicester: €51.8m

Leicester could have sold Mahrez for twice this price at the end of last season, but the winger's value has dropped after a poor title defence. At 26, he has time on his side, and Monaco or Arsenal might fancy him at less than €50m. But the fact he wants out, means they can negotiate.

Gianluigi Donnarumma, 18, Milan: €39m

Donnarumma is something special. At 18, he has already been playing for Milan for two years and he has revealed he won't sign a new contract. Given his current deal expires in 2018, his price will continue to drop, which makes him a bargain for Europe's elite.