If I may make a suggestion. You should try Adwords again. It takes some time to nail down the focus of the campaign, but getting it down to $0.01 per view is very much attainable (and what I currently get). With a good channel trailer and $0.01 per view, $100 is 10,000 views. And given my history, $5 daily spend returns 10-15 subscribers, so $100 is 200-300 new subscribers.
The other reason I avoid Virool and use Adwords is Adwords can focus entirely on YouTube viewers. Much higher liklihood they have a YouTube account. Anyone who doesn't have an account, isn't someone you want to spend money on advertising to.