I've heard different statistics from different people, but the ones that seems most likely is this:
1) Most internet savvy people will click on the natural listings more because the understand what sponsored listings are.
2) People (savvy or not) will use the natural listings more during the purchase research cycle, then when the time to buy comes, they'll use the sponsored listings expecting to be sent directly to thier desired product so that they can conduct thier purchase quickly and easily.
3) Stats I've come accross from various sources (conferences and the such) say that it is a 60/40 split in favor of the natural listings.
Wow, I thought it was like less than 3% that click on the paid listings. I'm not sure where I heard or saw that, and it was a long time ago, but if 40% of the people viewing a search results page click on the sponsored listings, this seems significant enough to make the investment.
Most of the people I have observed and questioned say they ignore the paid listings at the top of the page. Thanks for your input.
I think the percentage breakdowns are highly dependant on the search engine used (which takes into account design and audience). I've heard that the paid click ratio on Google is fairly low (unobtrusive ads), while going progressively higher on Yahoo, AOL, Ask, and MSN. I forget the exact numbers.