I've been working on a project with similar CPA targets:
My method: Through some research I identified the type of content the target audience loves.
1) I'm now producing Tofu content relevant to the audience and will be using paid distribution channels to draw visitors.
The landing pages have been set-up to capture emails but I expect a few people to give their emails after reading one piece of content.
2) The people who visit the website would be tagged and retargeted on Facebook and Twitter with a free ebook offer.
3)The first two steps are designed to fill my funnel with people interested in the ebook, I would use "lookalike audience" to target people similar to those who downloaded the ebook.
4) Compare cpa of those who read a post before being shown an ad to those who were targeted with the free ebook directly.
5) If all fail, go on fiverr and buy 10,000 "Highly Engaged" email signups and celebrate my accomplishments.