Osimhen had a good performance the last season and he was an effective player for Napoli because in the games they didn't have this player Napoli couldn't have a good performance just like the games they had against Ac Milan and he was injured. That's why Napoli is trying to keep this player for another season or make good money from selling this player. 150m is maybe a lot but not for a player like Osimhen and a team like Napoli.
150€ is a fair price for Osimhen because the quality of talent he has is so extraordinary that having such a high price is not a problem for any team that wants to sign him. Napoli gave such a big price because if management wants to be honest Napoli still needs Osimhen to keep getting the best place in the standings even though they always fail in the champions league at least being able to dominate in Serie A is much better.
I just want to guess if Osimhen's contract runs out I think Napoli will renew and won't let him go.