The CIES Football Observatory has released analysis on the deals conducted in the last window, which found PSG got the worst value of transfers in Europe's top-five leagues in landing Mbappe from Monaco in an initial loan that they will make permanent for €180 million.
Their algorithm, which "scientifically estimates fair transfer values" rated Mbappe, 18, to be worth €92.6m, meaning PSG will overpay by €87.4m in the deal, once add-ons have been taken into account.
Liverpool, meanwhile, landed the best bargain, according to the Observatory, with Salah bought from Roma for a cool €50m despite being valued at €69.4m.
Other teams guilty of overpaying were Barcelona and Manchester City. The analysis showed Ousmane Dembele, whose total cost could reach €147m, was added for a fee that was €51.2m higher than a fair value of €95.8m.
City, meanwhile, splashed out €57.5m on Benjamin Mendy, who was listed as a €28.5m talent.
Pep Guardiola's side were also on the wrong end of another deal. Of the sides to land the best bargains after Liverpool's move to add Salah, Sevilla's signing of Nolito from City for €10m — when he had been valued at €27.4m — came in second.
Chelsea's €39m move for Antonio Rudiger , who was rated as a €54.2m player, was third-most shrewd addition.
The CIES Football Observatory said: "As usual, a negative gap was measured between amounts paid and values estimated.
"On average, the former were 30 per cent lower than the latter. It is the biggest difference observed since the implementation of the transfer value algorithm. This finding reflects an acceleration of the inflation occurring in the transfer market."