Hypothetically, I'd wonder how much it would cost the Saudi Royal Family (who bankrolls their league) to buy like the 100 best players in the world and could they afford that? Lets say they paid the 100 best players $100 million/year, that's only a payroll of $10 billion/year which I'd think is pocket change for them.....and of course, you'd make some of that money back.
It wouldn't be enough simply because some number of those 100 players wouldn't play there for any price. Like I said, lots of athletes would rather be rich in a situation that they are comfortable than really rich in a situation where they are not. Lots of the best soccer players won't even consider playing for someone that doesn't guarantee them a chance to play in the Champions League, and if they go to Saudi Arabia they aren't going to get to do that.
By the way, you might not know this but for Mbappe, signing a $100 million per year contract with the Saudis wouldn't even really be a raise. He isn't going to Saudi Arabia for anywhere close to $100 million per season.