Even though you purchased a link on a high PR page doesn't necessarily mean you bought it to increase your PR. How can Google tell the difference? Especially if the webmaster doesn't have a payment link on their page stating that that's the purpose of having a link on their page.
its not that Google can detect paid links. Its most people are fools who think they can outwit Google so they hire link spammers to build 10,000 links in a month which is completely unnatural., They will buy only PR4 or higher links which is unnatural. All the links will come from related / relevant sites which is unnatural. Basically they are morons who thought they could outsmart CS Engineers from the top colleges and universities through out the world that Google hires.
So basically, if you choose to go that route you should "slowly" purchase high PR link to make it look natural?
Google can't distinuguish between paid links and normal links unless you use a tracking URL (given by the sponsored site) or you mention that in your post.
I knew it would not take long for someone to prove my point above. And you know Google cannot distinguish between paid links how??? Did you or do you work for Google??? Passing out misinformation does nobody any good.....
You dont need to work for Google to know that they cannot do it If you know how search engine works and the basics of HTML .. then you will know that Google will not be able to see more that what you can see, so if you hide your link well and the link was really related to the page content it is impossible for even a human being to know if it is paid of not. Here are some patterns to avoid: 1- Dont place links in your footer. 2- Don't use words like paid or sponsored around the links. 3- Don't link to irrelevant websites. 4- Use contextual links 5- Don’t use site wide links. Finally I need to remind you to worry about humans more that robots .. I am sure that thousands of people using these form on daily basis: http://www.mattcutts.com/blog/how-to-report-paid-links/
Ahh well I have no clue how search engines work....there's the whole problem.... Guess I should go pull down all the # 1 positions I have on Google...... Ohhh and anything that comes out of Matt Cutts mouth is half fantasy and half fiction.....