Many companies that sell and broker text link ads are ripping people off. They are selling single links on high PR websites, leading you to think that high PR is all that is needed to rank well in the search engines.
This is untrue.
PageRank has very little to do with rankings at all, and Google has made it completely unreliable even for determining which websites are more valuable than others. Therefore, you should not use PageRank as a guage for a web page's importance when deciding whether or not to advertise on a particular site.
Instead, you must look at numerous factors, such as traffic, quality content, uniqueness, target market, et cetera.