Well 120 Million € is a huge amount of money, but that is also the current market value of Osimhen and he also has 2 years worth of contract left at SC Napoli. So it just makes sense that they are asking for a transfer fee, that his around his current market value in my opinion, even though that amount of money for a single player is just crazy, but that is just they way modern football works at the moment.
The move from Enzo Fernandez was totally different story though in my opinion. FC Chelsea paid 121 Million € for him even though his market value is "only" 80 Million, but back then Chelsea and their new owner was just throwing around money without any reason and without brain. Now they have a squad that is way to big.
PSG bought Luca Hernandez, who is injured and hasn't played for a year, for 45 million euros.
Plus Osimhen is one of the most gifted and brightest players at the moment. He has not just a talent for the game, but also a great fighting character, he always escalates the game at the enemy's goal and his goal sense is excellent. He helped Napoli a lot last season and even with a price tag of 120 million euros his purchase would be a great investment.