I am not sure what the typical visitor numbers are for a site with a page rank of 5, but can anyone guess how much a top banner space for one of these sites would be for say one month? I am asking because I see people selling banner ads in the advertising service section of DP for 20 dollars a month. Some have page rank 4 others have page rank 5. But then I go to Sohh.com and their PR is 6, they tell me I better have a MINIMUM budget of 500 beans. If anyone has a graph that correlates a sites visitor statistics with the value of their banners for a month, that would be great.
it will depend more on the website traffic then pr for banner links, also targeted traffic is essential.
See banners drive more traffic to your website than a normal text link. You will get dozens of pr 4 to pr 6 sites selling links at $10 a month. But if your requirement is to drive traffic to your website text link is not a good Idea(still it depends on site traffic) else if you want to increase your SERP and page rank you can go for Text links. Coming to the point how can we justify which banner is really worthy? Check alexa of that site you will get a rough idea else check site popularity on google ,yahoo and msn.
You can not directly link the PR and the traffic, they have nothing to do one with the other. Traffic may be high while PR low and the other way around, while the price of a banner is as high you ask. The good way to find out is to measure the CTR and the eCPM with AdSense , to see results and then to sell advertising. Ask the people selling advertising which would be the CTR and eCPM and maybe ask for some reports. This way you can know if you throw your money out the window or not.