Study: Courtois the biggest bargain of summer transfer window

Chelsea lost millions on the sale of Thibaut Courtois to Real Madrid and overpaid to sign Kepa Arrizabalaga as the Belgian's replacement, according to the CIES Football Observatory.

Factoring in each player's reported price tag and their corresponding market value, the research group determined the best bargains of the summer transfer window and the most overpriced.

CIES estimates Madrid signed Courtois €22.6 million below his market value, while Chelsea coughed up €44.7 million more than they should have to recruit Kepa from Athletic Bilbao.

Here are the most cost-effective operations and most inefficient deals of the window:

Signed below value

Signed above value

The presence of a release clause either significantly reduced or increased the cost of certain signings. Atletico Madrid and Barcelona signed Rodri and Clement Lenglet at a discount from Villarreal and Sevilla, respectively, while Chelsea paid handsomely to meet Kepa's €80-million release clause.