Manchester United's record 59.7 million pound fee paid to Real Madrid for Angel Di Maria was the most over-priced transfer of the summer window, according to a study by the CIES Football Observatory.
A report by the Swiss-based company revealed that major European clubs paid 16 percent more for players in the transfer market this summer than those with similar profiles five years ago.
Di Maria's move to Old Trafford was adjudged the transfer most affected by football inflation while CIES also claimed Paris Saint-Germain paid tens of millions of euros above market rates for Brazilian defender David Luiz -- who they signed from Chelsea for 62 million euros (50 million pounds).
"Our new data analysis has revealed that overall, clubs paid on average 16 percent more than they invested in the five previous years for players with similar characteristics," CIES said.
"This confirms the ongoing inflation trend of the transfer market at the top end of the football pyramid.
"The inflation trend is mainly due to the sums spent by a handful of wealthy clubs ready to pay more than a player is worth to get the best talent."
CIES analysed transfer activity from Europe's "big five" -- the English Premier League, Spain's La Liga, the Italian Serie A, Germany's Bundesliga and the French Ligue 1 -- and said that Manchester United paid the most inflated fee, deeming Di Maria's purchase to be 24 million pounds over his market value.
PSG were estimated to have overpaid by 23 million pounds for David Luiz while James Rodriguez, who moved to Real Madrid from Monaco for 80 million euros (63 million pounds) following his Golden Boot display for Colombia at the World Cup, is said to have an actual market value of 55 million euros.
At the opposite end of the spectrum, Liverpool's acquisition of Mario Balotelli from Milan for 16 million pounds (20.2 million euros) was regarded as the most underpaid fee of the summer, with the Italian striker's actual market value rated at around 36 million euros.
Balotelli was signed by Brendan Rodgers to replace controversial striker Luis Suarez at Anfield and the Uruguayan's 75 million pound (94 million euros) switch to Barcelona was considered another cut-price deal by the CIES -- a conclusion attributed to "the existence of buy-out clauses and/or non-sporting related issues."
The third most under-priced signing of the window was Spain international striker Diego Costa, who moved from Spanish champions Atletico Madrid to Chelsea for 32 million pounds (40.5 million euros), a difference of 10 million euros to his actual market value.