The price of Osimhen has decreased. If you take a look carefully, his previous market value was €120m, it is only €100m now. I think the price of Osimhen can increase again if Napoli can perform better in the next season. I assume, Napoli can finish in UCL zone under Conte's management. 
transfermarkt
After Spalletti left Napoli, Napoli never played with their best level again. Only Spalletti who knows well how to manage Napoli players. It may be caused the squad of Napoli was built by Spalletti, so it is developed around his tactics. The new coach of Napoli may need to adjust their tactics with the capability of Napoli current players. Antonio Conte must be aware with the capability of Napoli current squad, he mustn't be egoistic to apply the tactics.
I mean it was obvious that it would, was it based on his talent or was it based on correction? Because let's face it, value is not always about how much that player worths, it is also based on how much that player worths to the team that owns him. For example, last year Napoli won the title, and Osimhen scored a lot of goals during that period, and yet this season he didn't and they didn't.transfermarkt
After Spalletti left Napoli, Napoli never played with their best level again. Only Spalletti who knows well how to manage Napoli players. It may be caused the squad of Napoli was built by Spalletti, so it is developed around his tactics. The new coach of Napoli may need to adjust their tactics with the capability of Napoli current players. Antonio Conte must be aware with the capability of Napoli current squad, he mustn't be egoistic to apply the tactics.
This means that last year he was worth a lot because he was the scorer for the Serie A champions, this season he was the player that had a lot of issues with the management and barely played on a team that didn't do much all season long. So, it is not just about skills, it is also about the fact that player may worth more than he should or less than he should to teams.