I think the reason for the drop in Osimhen’s market value will be because of the issue surrounding his move to Galatasaray and the kind of deal Napoli is offering. I see no other reason for the drop because despite the circumstances around his move he has being performing well for Galatasaray. It is good news that Napoli are finally deciding to reduce his price tag, this will make it easy for interested clubs to make a deal for his services in the summer.
Regardless of the beauty to Galatasaray or not I think one of the reasons is because of the main problem in Napoli and the mistake of the management itself who held Osimhen for too long even though the previous 100 million was still possible even PSG could pay a little more but the chaos occurred because Napoli seemed to think that it was not very valuable and preferred to loan rather than sell directly.
The previous main choice was clear when there was no good relationship between Napoli and Osimhen they should have moved quickly to sell but they tried to hold back even though the situation was critical.
Currently the price drop is happening and that is natural especially the hype from Osimhen's performance is not as visible as what happened at Napoli even though he did not play badly at Galatasaray either.