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France international Kylian Mbappe scored 26 goals in all competitions for Monaco last season

European clubs paid on average 30% more than a player was worth during the recent transfer window, a study says.

The CIES Football Observatory, external-link using a transfer value algorithm, claims Paris St-Germain's proposed 180m euro (£165.7m) deal to sign Monaco forward Kylian Mbappe was the most overpriced.

It says the fee was 87.4m euros (£80.4m) more than his estimated worth.

The study also suggests Manchester City overpaid by 29m euros to sign Benjamin Mendy from Monaco for 57.5m.

The Football Observatory uses various statistics, including age, goals and points won by the team, to calculate a value external-link for a player.

Greatest gaps between fees paid and estimated values (summer 2017) Player Move Estimated (euros) Actual fee (euros) Gap (euros) Kylian Mbappe Monaco to PSG 92.6 180 +87.4 Ousmane Dembele Borussia Dortmund to Barcelona 95.8 147 +51.2 Benjamin Mendy Monaco to Man City 28.5 57.5 +29 Jordan Pickford Sunderland to Everton 6.8 34.3 +27.5 Gylfi Sigurdsson Swansea to Everton 24.1 49.3 +25.2 Mamadou Sakho Liverpool to Crystal Palace 5.3 28.2 +22.9 Patrik Schick Sampdoria to Roma 21 42 +21 Nemanja Matic Chelsea to Man Utd 30.2 50.3 +20.1 Mohamed Salah Roma to Liverpool 69.4 50 -19.4 Harry Maguire Hull to Leicester 3.1 21.6 +18.5 All figures from CIES Football Observatory

Goalkeeper Jordan Pickford, 23, left Sunderland for Everton for 34.3m euros (£31.6m) in the summer was called up to the England squad last month.

However, according to the Football Observatory, that fee was 27.5m euros (£23.5m) more than Pickford's estimated worth.

The study also said Manchester United overpaid by 20.1m euros (£18.5m) to sign Nemanja Matic from Chelsea, although former England striker and BBC pundit Ian Wright described it as the "last part of the jigsaw" for United.

Mohamed Salah's 50m euro (£46m) move from Roma to Liverpool was less than the forward's estimated value of 69.4m euros (£63.9m).

Greatest gaps between fees paid and estimated values in Premier League (summer 2017) Player Move Estimated (euros) Actual fee (euros) Gap (euros) Benjamin Mendy Monaco to Man City 28.5 57.5 +29 Jordan Pickford Sunderland v Everton 6.8 34.3 +27.5 Gylfi Sigurdsson Swansea to Everton 24.1 49.3 +25.2 Mamadou Sakho Liverpool to Crystal Palace 5.3 28.2 +22.9 Nemanja Matic Chelsea v Man Utd 30.2 50.3 +20.1 Mohamed Salah Roma to Liverpool 69.4 50 -19.4 Harry Maguire Hull to Leicester 3.1 21.6 +18.5 Davide Zappacosta Torino to Chelsea 11.8 30 +18.2 Antonio Rudiger Roma to Chelsea 54.2 39 -15.2 Kyle Walker Tottenham to Man City 41.3 56 +14.7 Alex Oxlade-Chamberlain Arsenal to Liverpool 30.5 43.4 +12.9 All figures from CIES Football Observatory

Total change or tweak to the transfer system?

Neymar's 222m euro move from Barcelona to PSG in August is the biggest transfer in history

According to the Football Observatory, clubs in Europe's big five leagues (England's Premier League, Spain's La Liga, Italy's Serie A, Germany's Bundesliga and France's Ligue 1) spent a record 5.9 billion euros (£5.4bn), a 41% increase on the previous year.

Following Neymar's 222m euros (£204m) move from Barcelona to PSG, world players' union Fifpro criticised the "anti-competitive, unjustified and illegal" transfer rules governed by Fifa and called on the European Commission to investigate transfer fees within the European Union.

The Football Observatory is also keen for the distribution of transfer fees to be addressed, suggesting an increase in indemnities paid to the clubs that develop players.