Should you purchase high PR links or buy many lower PR links? We get this question all the time. The answer is the same as with all text link ad questions - it depends. It depends on several factors like your site's age, current backlinks, industry, and page relevancy.
If you have a brand new site, early on you are probably better off buying relevant lower PR links at a steady rate each month rather than buy one super strong link to a brand new site. Why? A brand new site is very unlikely to get links from PR8 plus websites. It just doesn't look natural and may raise a flag with the search engines. However, if your site is somewhat established and you have plenty of backlinks, a few high PR links can put you over the top for your keyword phrases.
High PR Links to Get Spidered
New or old website, one thing purchasing a high PR link will do for your site is get it spidered quickly and deeply. So it might be worth it to buy a high PR link for a few months to get your site more fully indexed. In this case, one nice PR9 will do the trick. Unless you're loaded with cash, you don't want two or three PR9s pointing at a new site to get indexed - use that money to buy 20 or 30 low PR links (preferably contextual links) each month.
High PR Links as a Last Resort
There is one other scenario where I would recommend buying high PR text links: If you have a quality website that is doing nothing in the SERPs it might be worth a shot to buy a bunch of high PR links to see if you can 'prime the well' and get your site ranked. If your website is not ranking, you really have nothing to lose and 3 or 4 high PR links might get your website out of the Google sandbox. If it doesn't work, well, you still have the bad rankings you always had so no real harm done (except to your wallet).
Remember, LinkAdage always recommends that link buyers should not focus entirely on PR. Link smart and link natural - get a few nice PR links and then shoot for totally relevant contextual links for the most success. Google loves long term relevant links, and in the end, they will most likely help you much more than a bunch of random high PR text links. If the page where your link is at it is indexed, cached, and relevant, it is a very valuable website link.