I agree 400 million is too expensive for the price of Osimhen. His performance is decreasing this season, his price should be lower than the previous transfer window. Man United probably won't spend 400 million for signing Osimhen, they can get more than 4 star players with that money. Even Saudi clubs won't have the intention to sign Osimhen with 400 million. It is just nonsense to set the price 400 million. 
I'm sure Mbappe and Haaland must be above 200 million if he has a contract with their current clubs. But Mbappe seems to be free agent because his contract expires in PSG. Well, if Napoli sets the price 150 million, I think there is a chance for selling him. Real Madrid, some top teams in EPL, and Saudi clubs are probably interested to negotiate with Napoli.
I think it was just made up by the media by saying 400 million I don't think this will happen for the sale of Osimhen because this player is not his level at that price, it also doesn't make sense and who wants to buy him? If you say this is nonsense then I believe it.I'm sure Mbappe and Haaland must be above 200 million if he has a contract with their current clubs. But Mbappe seems to be free agent because his contract expires in PSG. Well, if Napoli sets the price 150 million, I think there is a chance for selling him. Real Madrid, some top teams in EPL, and Saudi clubs are probably interested to negotiate with Napoli.
Mbappe does have a high value at the moment the contract at 200M is still very possible especially now that Real Madrid is still eyeing him until his contract expires. Let's see how Real Madrid make a special offer to Mbappe.
I don't know if the 150M for Osimhen will go to Chelsea this is just a rumor while Real Madrid will not prioritize the Napoli player, while for the Arab club Osimhen there is no interest there.