does google give their highest paying clicks to the pages with the highest pagerank? or most traffic or is it completely random for all sites no matter what?
I dont think that PR has anything to do with high paying clicks. It depends on Niche, some are high paying, where some are not.
None of your stats has any thing to do with it. It is completely random. You just get ads that are relevant to the information on the page. The amounts you get per click depends on the advertiser. If the ads that appear have a lot of competitors you will get a high price for that click.