PR is not the factor that determines your earnings! The factors that determines your adsense earnings are: a> Niche of your site. b> Traffic c> Traffic type (like they are from search engines / adwords / traffic exchange) d> Ads placement and optimization
PageRank has nothing to do with earnings of the site. Earning of site more depend on traffic, where does this traffic come from, content and keywords.
PR doesn't indicate traffic either. I have a PR2 site with maybe 20 hits a MONTH, and a PR 1 site with about 3000 a day. I'm still fond of my PR 0 site with 20 impressions and 8000 visitors a day.