I wouldn’t consider that a source of information, it’s just a theory. PIF paid around 43 million euro in total for Zeyach, Mendy and Koulibaly. That’s a reasonable amount for these players.
The transfer amount hasn't been officially disclosed, but the rumored 20mGBP for Koulibaly surely isn't a reasonable amount. He was an unwanted, grossly overpaid 32yo CB, apart from Saudi clubs the best Chelsea could hope for was a loan with an option and a small fee. When we were discussing Koulibaly potentially joining Juve, everyone agreed that it only makes sense if Chelsea pays some of his salary. 18m for Mendy, a 31yo reserve goalkeeper is a bit pricey too, but not that suspicious.
I don't think Ziyech has moved officially yet, it remains to be seen what the amount will be. However he's a 30yo winger with 1000 minutes played for Chelsea this season and 0G3A.
All in all the Saudi did Chelsea a big big favor here, I'm still keeping my fingers crossed we'll see some Saudi club pay actual money for the likes of Alex Sandro, Bonucci and Pogba, but we might not have the right connection sadly.