For the strength and level of their league, they do have a fair squad, AlnHilal happens to be one of the top five teams so they are doing well domestically even if it is not beyond Saudi Arabia. One of the reasons you see then going for top European players is to enable them improve the quality of their league because they know with the players they gwt from their domestic leagues, they can hardly get the same kind of quality they will get with the European players.
They have targets and ambitions by bringing in these expensive players, apart from improving the quality of the league they also want to attract the world's attention and so far it's working because almost all of us are talking about this league. The main highlight is that Saudi Arabia wants to host the next World Cup and I wonder if that target is achieved will they stop bringing expensive players to the league? Obviously no one wants to spend a lot of money without a target especially if the target has been achieved.
The Saudi pro league has taken the world of soccer by suprise overnight and transformed into something everyone now wants to talk about. Their business strategy is simple, buy the talking point and get more fans. Before the signing of Christiano Ronaldo, the Saudi pro league has been as good as unknown but Ronaldo alone brought a huge publicity to them and raised Al Nassir followers to a whooping number that people gradually seeking updates about the league. Ronaldo's move to the league opened the door for many great European players to start moving to the league, some for financial reasons and others for personal reasons too. The both parties are happy, the league is growing up fast while the players are making it big. Since Ronaldo, many players have joined the league boosting their income and publicity too.