The 21-year-old midfielder is seen as a possible replacement for Cesc Fabregas, who has left for Monaco.
He believes the player has not yet shown he is worth the reported price tag.
"Unfortunately today football is going in a very wrong direction," Cassano told Sky Sport Italia.
"He is young too, he is 27 years old, he is worth at least 4-5 times more than Barella.
"A player makes two good games, he runs right and left, maybe he is also a player of quantities like Allan or Barella.
"And then he is worth immediately 50, 60, 80 million? For me it is madness."