To get PageRank 5 when you have PageRank 3 or PageRank 4 is much easier than to jump from PageRank 5 to PageRank 6. So the higher PageRank is the more difficult it is to get it and it’s wrong to think that if you have bought 50 links to get PageRank 2 you need 300 more for PageRank 8.
For real the PageRank value is not what we got used to see: from 0 to 10. It’s used for the approximate marking. Google operates quite different values of PageRank in its algorithms.
PageRank is only one of the many factors affecting search results. A high PageRank doesn’t guarantee you the first position in search results by any request.
The PageRank value is affected both by external and internal links. So the internal relinking is important!
Site content and its updating don’t affect its PageRank.