Advertisers pay a higher price for better times (ie.. primetime tv, superbowl) or space (ie.. better located billboards). The same thing happens online, advertisers pay higher prices to appear next to the popular videos or websites.
I couldn't find any actual numbers, but it does seem that being popular on Youtube can pay out reasonably well... http://www.nytimes.com/2010/09/03/t.../03youtube.html
Originally Posted by 'New York Times Article
YouTube shares advertising revenue with content partners, who may be big entertainment companies like Lions Gate or amateur videographers who have developed a following. Hundreds of these partners make more than $100,000 a year. Some, like Sal Khan, a former hedge fund manager who now makes math and science education videos, have quit their day jobs.