What if you had a site that had absolute rubbish or no content, little or no traffic and no income but, somehow, had a Pagerank of 5 or 6, what would it be worth, if anything? 

Pagerank is a measurement of the back links that point towards any site. It is an arbitrary measurement that only gives information about that and nothing else. It’s often true that the sites you see in search results have high Pagerank. If you are looking for higher search engine rankings a high Pagerank may or may not help you. If your field is highly competitive you will need a lot more than good PR but if it is less competitive a Pagerank of 5 or 6 will generally give you a good ranking till it’s the nest Page rank update. 

It is confusing between a high Pagerank site and a quality site. A PR8 site isn't necessarily a quality site and consequently a quality site doesn’t necessarily have to have a high Pagerank. If you are after links from say link exchange, a high Pagerank will do you good as most quality sites will not do a link exchange with a site having under a PR4, also if you are wanting to sell advertising space a lot of untrained webmasters only look at the PR and think wow a PR5 site must have good traffic and will by an ad spot without doing any research first.

In general the higher Pagerank site (all other things being equal) will probably have a higher worth than the lower Pagerank site, but again that isn't always going to be the case and for me it's the totality of a site or a domain that determines it's worth not any one thing. But a good Pagerank alone doesn't add worth to a site. There's a lot that goes into determining a site's worth and most of it won't be based on Pagerank. Remember, Page Rank is important but is not everything. Pagerank, solely, is only important to a site if you’re planning on selling text links or something similar. Other points such as good content, good amount of traffic, sales generated etc also need to be taken into consideration along with good Pagerank. A high Pagerank site will probably be worth something to anyone who doesn't look beyond the Pagerank to decide if the site is worth something, but for people who are actually more concerned with your traffic and the quality of your traffic as well as the quality and revenue generating potential of the site it's probably not going to be worth a lot.