Look at that in all honesty he's worth them all, if you look at how many goals he's scored. Just to mention, Benzema Ballon d'Or has scored fewer goals than him, in an easier league (in the sense, scoring is easier in LIGA)
So they are right to sell it at that price.
Napoli's management really understands that Osimhem is worth keeping for next season, it is impossible to get players at his level in the transfer market this time. If released, of course, Osimhen's replacement will need time and support so that he can be at his level at least one or two seasons for sure. If Napoli let Osimhen go for 150 Million Euro it is very reasonable, I agree with your opinion because Osimhen is fully tied to them for the next two years.
Napoli understand very well the current transfer situation if a club dares to bid at the price they want then Osimhen will be released. From the other side, the reason why Napoli maintain Osimhen next season i think they want to defend the Scudetto title, it will be very difficult to achieve it without a solid squad and sharp strikers. Holding him for one more season certainly won't affect Osimhen's price considering that he is currently only 24 years old.