There is no guarantee for Osimhen to play in the Saudi Pro League at this very young age, no matter whether Osimhen really prioritizes about money or more prioritizes about his great career in the big European leagues. Because after all, playing in the top European league also Osimhen will still get a lot of money, and playing in Europe is also not about getting a lot of money, but will also make his football career better at a young age. Thus, it is not right to choose to play in Saudi at such a young age, because football is not only completely about money, but also young players still need great experience to play in the top European league.
Osimhen is 25 years old. He isn't very young age. 
Sure, Osimhen should prefer his career over the money. He is still in the prime age to play in big European leagues. Since he performed well with Galatasaray this season, there must be many offers from big teams in Europe to sign him for the next season. So, it won't be so difficult for him to move to a new team if he doesn't have the intention to stay with Galatasaray.
I've the same perception that he will only consider to move to Saudi league when he is no longer in the prime age. It may be above 30 years old, it is still around 5 years from now. So, it will be too early if he considers to move to Saudi league now.