I think HitWise's post about determining the optimal amount of PPC vs. Organic listings is interesting (for a change) subject but the blog post did not answer the question. I liked the "bubble" diagrams that were used in the post.
"…In answer to the question in the title of this post - "what is the optimal balance" - it depends. It depends most on what works for you. Test, monitor and adapt constantly and continue with the tactics that yield a positive return on investment. It also depends on the structure of your website (how search engine friendly is it?)."
In this case…I'd like HitWise to come up with something beyond what I already knew….testing, monitoring and adapting tactics depending on your goals and website is not noteworthy by itself.

I'm not sure if PPC and Organic Search Traffic are mixed in the chart above or not; I'd have included two charts for each site - the bubble diagram for Organic Search traffic (they don't seem to have one for that) and the bubble diagram for PPC Traffic (above).
Otherwise, I liked the post - I think there is an optimal value of PPC vs. Organic and would like to know what it is or could be for each type of site.