The positional information of clicks from Google
Looking at the "position in search results" data in Google Webmaster Tools will only tell you a slightly deceptive number of the actual position of the search term at the point of it being clicked.
The reason is that any search term that does not muster 10 clicks or more will be reported as "<10" in the report. Once tracking the positional data is activated in any decent web analytics solution a far more accurate number of clicks per search term can be extracted.
Note that the data on search terms and position in the SERP is lost when users use Google while logged in and use secure search (https). The referrer information only provides information that Google is the referrer.
Top 6 in SERP
Having a result position of 6th or better gives a click share of ~85% of all click referrals from Google. Essentially those typically are the positions above the fold, i.e. visible in the SERP without the user having to scroll down.
Anything beyond the 10th position falls into the small 6 - 7 percent of Google clicks, so the target when paying for SEO should be that the search term lands in the SERP on 6th position or better if to provide any volume.
Why else pay for SEO?
From a conversion perspective this might vary, the focus here is pure click volume.
Popular search term movement
As can be seen in the picture, a shift of volume from the 1st position to the 4th is quite evident, this caused by a slide of a few popular terms loosing SERP position. And yes, it's all organic. Despite the drop in position the click volume has remained the same and the changes of which words rank #1 or not has had little impact to the click percentage of the top 6 search terms.
Search terms with links to a site appearing in the top 6 search terms of Google SERP accounts for a majority of the click volume. The metric of how large a search engine index is isn't in reality relevant, users' click to a high degree only on results in the top 10 position on a relevant SERP.