On Monday, Coutinho officially completed his transfer to Barcelona on a five-and-a-half-year contract, and figures of between £142m and £146m have been reported in the media.
However, when quizzed on the money spent on the Brazilian international, Mestre claimed that he was unable to disclose any further information.
At a press conference, Mestre said: "If I said anything it would break the pact we have with Liverpool. I don't like it but I cannot give any number."
Back in 2013, Liverpool paid just £8.5m to sign Coutinho from Inter Milan.