According to online advertising network Chitika, a lot. The company took a sample of 8 million impressions across its network, and found huge differences in clickthrough numbers between the top few positions, as well as an even greater one, percentage-wise, between position 10 and 11. Now, this won’t come as a surprise. But it does reinforce the need to keep working on those Google results positions, especially by getting external links to your site, which has the double effect of creating direct traffic and boosting your Google ranking.
Full story in The Value of Google Result Positioning.