$20 eCPM isn't a problem, I once had a site that was getting like $50-60 ecpm (I won't mention the niche, but what I noticed is that the people who clicked were coming from short-tail search phrases on Google, for example, if someone came typing 'learn how to swim' and the adsense ad title on my page was 'learn how to swim' they would click on it). Scent...what you type is what you click.