The 80/20 Rule

Having just had a bit of a discussion with a client about traffic for their site. Namely, what search terms are converting traffic, once you take out any references to the company name, I thought it an interesting topic.
You would expect any SEO campaign to focus on a core number of search terms with which to promote the site. My client has been complaining that those terms are not converting into visitors very well, that if you divide the number of visitors arriving at the site under those specific terms with the amount spent per month - it equates to a hefty CPC.
Well it does if you look at it like that :-) However what the client doesn't seem to appreciate is the other terms that are bringing in traffic to the site. You'd never expect to just see traffic to the site based solely on 5 terms for example. The Long tail principle, comes from the 80/20 rule. 20% of the traffic to your site is coming from those specific terms whilst the significant 80% is coming from related terms. The total number of low-number referred terms outnumbers the total of top 10 terms.
Once you bring in this 80%, the CPC looks alot healthier.