I would like to inform everybody that page rank is a logarithmic value. for every integer of PR rise you are outperforming 10x as many website that have one rank lower than you. this conversly means that pr5 is 10x as hard to obtain than pr4, pr 7 is 1000 times as hard etc. somebody correct me if i am wrong.
Where did you get the x times concept from ? Conceptually you are right that the next level might be x times harder to achieve but it is rather not possible to arrive at the approx number of times.
I believe its from googles own engineers. Would explain why there are only a handful pr9 sites and zillions of pr6 sites