PR is the best metric we have for keyword difficulty.
1. It is not the PR of the page that matters as much as the PR of the website and the only useful metric we have for this is the HomePage PageRank.
2. When the HomePage of a website is competing for a keyword, Google boosts its effective PR to about 4.5. In a study of 5,000 keywords (50,000 top webpages on Google) 94% had HomePage PageRank of at least 4 (64%) or they were HomePages (30%) -"Domain Authority: The Top Two Factors On Google: A Study of 50,000 Webpages in 5 Countries". High positioning for keywords depends not only on the PR of the HomePage (website authority) but relevancy which depends on page content and the anchor text of incoming links.
If your website has low HomePage PageRank compared to the webpages on the top Google search results page for your preferred keywords, you have virtually no chance of top page positioning. HomePage PageRank is the top metric we have for keyword difficulty. A website should target keywords that are in range for its website authority and gradually build its authority so that more difficult keywords come into range.