It depends how you count a user interaction. YouTube ads are pay per view. A view is defined as someone watching the ad for 30 second or longer, at which point the advertiser gets billed for a view (around 10 cents or so). However, only a small % of these views lead to a click to the advertiser's website. So although the PPV is lower than contextual CPC, it is about the same when one compares actual traffic to the website.