It's not just PR that decides the rate. Traffic on the site, niche as well as position on the page will also matter. You should check the link sales section to see more trends.
it depends on traffic and quality of sites, as well as unique constant of particular blog and PR. BUT only pr is not the whole factor.
To give you a ballpark figure, I sell links on some of my sites in the current price range: PR4 sites - anywhere from $20 per month to $50 per month depending on the page and the page rank of that individual page where the ad runs PR5 sites - anywhere from $50 to $300 per month, again depending on the strength of the page Dan
I think niche is another important consideration. Just like keywords, some niche's are far more expensive and other are cheaper.
The number of outbound links on the page is a factor that we consider to be very important. We buy links mainly for the SEO benefit, and not for traffic. Traffic is almost irrelevant to us, but then I do things a little differently.
when I look to rent a link, I look at alex, how often the site is cached, pr, quality, and theme of the site before purchasing. Links for finance are far more expensive that for calculators. I have many links for sale and rent. Let me know if your interested. PM me.
It depends on niche, traffic, and backlinks as well. Check out the link sales portion of DP and see how much people are selling things for.