Cristiano Ronaldo is a superstar at the Al Nasser club because of his stay at the club, the club's reputation is prolonged. Because he was the first European player to join the first Saudi ProLeague. Since then a number of stars have been joining Saudi ProLeague clubs, so I think the contract will increase as he teaches the game to other players how to attack and how to defend.
I think Ronaldo isn't the first European player joining Saudi Pro League. There are other European players who already played with Saudi clubs before the arrival of Ronaldo. Talisca is one of the European players who joined Saudi club before Ronaldo joined SPL. He joined Al Nassr in 2021, he ever played in Benfica and Besiktas when he was in European league. However, it is true that Ronaldo is the first European superstar who joined SPL.
Mbappe has declined any negotiations with clubs from the Saudi Professional League during the last transfer period when he refused the offer over 700 million dollars
We can understand if he rejected all the offers from Saudi clubs. He is still young, he still has a long career. He must prefer to stay in most competitive leagues in the world. He only can play in most competitive leagues if he play in the leagues in Europe. He still has the intention to get a better career as a footballer, that's why he prefers to stay in Europe.
Messi won't go to the Saudi League as he has no reason to do it. He earns enough money in the MLS due to his contracts with Apple and Adidas.
Messi has no interest to join SPL. If he wants to move to SPL, he already moved to SPL when Ronaldo joined Al Nassr. At that time we heard that there were Saudi clubs made amazing offers for him, but he rejected the offers and joined MLS.