Maybe they estimate his real value as 150 million and consider it a profitable transaction (expect to sell him in the future)? Otherwise, it is difficult to explain this record transfer for Turkish football.
A player hated by the fans, unwanted by the coach, even hated by the president. It just didn't make sense to keep him, unfortunately.
It's clear that Italy is losing a talent. If he had gone to Juventus he would have stayed in Italy, but staying at Napoli was now pointless.