The correlation between website traffic and revenues is variable, mainly on what type of people are going to your site and what kind of business are you talking to. For example, I used to work for Jewelry Television, and I noticed that they are advertising on more progressive blogs, which will tend to attract a certain type of audience.
Then, what type of traffic do you get, do you have pages where people will be there for a while, perhaps solving a puzzle, or carefully reading something, or is it just to click-thru. On click-thru sites the ad rate will be lower as there is less chance people will click on ads.
You can look at what Google Analytics tracks to get an idea what you may need to be able to estimate, to help with the guess.
pclark does have a good idea though about the three scenarios, as, he is correct, we don't know what the economy will do, and so we have no idea what the advertising budgets will look like, so you have to cover your bases, to show that in the worst case you have a plan on how to not go completely bust.
You will also want to get an idea as to what the percentage of each browser type is, going to your site, as some browsers are more likely to have ad-blockers, which will reduce the views those ads will actually get, which will also impact your bottom line.
So, there is no real equation that can accurately tell you for any arbitrary site what the correlation between visitors and ad revenue will be as there are so many variables that it would be at best a guess, and most likely a horribly bad one. You may be able to find an equation for certain types of sites, given a particular target market, and this is what a marketing person should be able to do.