How many expensive players have turned out to be duds under him at City? Can you name them?
Mangala was bought before Pep came to City, and I don't remember him being trusted by the previous coach either.
Also, the price of a player is not really indicative of how good the player actually is. Players are priced high for a lot of reasons such as age, transfer close, who the buyer is, etc... No one was calling Diaz and Stones world class players before they played for Pep. The same goes for Rodri, Ake, Grealish, Alvarez.