Depends on how good the site is. how good is the content and how often are the pages indexed by Google
I usually charge about $50 for 3 links that I insert into one of the old blog content that I've written already. These blog post have an average of 500 words and are written in a way that are generic and easily tweaked to fit around the advertiser's product or keywords. On blogroll, that's $20 per links. You also have to remember that the site has a PR of 6, so it's money well spent.
PR shouldn't be the only factor you use in determining the price. Number of outbound links, number of Google backlinks, and age of domain are all important factors as well. For a PR4, I'd put it between $5 and $20 per month, depending on those other factors. For a PR5, I'd put it between $10 and $30.