Check mustwatchVideos.com. I started it 4 months ago and is now a pr 2 site. I created this out of passion for learning something new. So now no plans of having any ads.
Regarding input cost -
Using drupal. Except the domain name and hosting, there is no extra expense. The modules are free and the theme too is free. Since the videos are on youtube, vimeo etc, the bandwidth is theirs. So you can use a basic shared hosting.
Returns -
I've not kept any ads, so cannot say anything. But from my experience in browsing other video sites, the chances of someone clicking the ads are less on a video site than on a text based site. Specially if the ads are text links. But there are more chances of someone clicking a banner ad or flash ad.