I really can see this happening as it will tick a lot of boxes for a number of parties.
Firstly, PSG get to save face by selling him for a huge profit to a team in a league they won't face. The Saudi's will get a player of genuine world class ability and it will send a huge message to Europe about their league.
I think Mbappe will be tempted by a salary that is beyond anything he could imagine. By going from Monaco to PSG he showed he was more about money than anything else so I suspect this would could happen - plus he's likely to set a record that won't be broken for a very long time. I wouldn't be surprised if there's a clause that he stays for up to 2 seasons before they let him go back to Europe (Real) at a reduced fee.
I think the difference between what China did and what Saudi are doing is that China got a few decent players, but the Saudi clubs are hoovering up anyone at will. It's astonishing to think that they're buying Liverpool's midfield and Liverpool are pretty powerless to stop it. I'm not saying they are world class but players like Henderson, Fabinho, Kante, Benzema, Neves, Koulibaly, Mendy, Milinkivic-Savic etc are the type of signings that a club who want to win the CL MIGHT make. These are just the ones that I can think of but it's really something to think about.
If they sign Mbappe, it would be absolutely astonishing.