How to project the potential traffic opportunity (potential visitors) from organic search listings?

When projecting the size of the available opportunity in search marketing, is it reasonable to assume that the PPC AdWords opportunity size in clicks/day on Google represents only the PPC portion of the search opp and that a much larger opportunity exists on the organic side that is perhaps more difficult to project?

Is it reasonable, for example, if an existing paid search campaign on a specific product has a maximum opportunity of 1,000 clicks/day and the ads have been getting a 2% response rate, that the total size of the opportunity “may” be up to 50,000/day (the other 98%).

How are folks projecting the size of the opportunity in Organic?

Thanks in advance for sharing your thoughts!

Adam
http://twitter.com/AdamBoettiger