I run growth.email (http://growth.email) which is a weekly growth marketing email, listing 10 articles in each issue. I started it at the beginning of this year, have around 1,700 subscribers and make $40 a week from one single ad in each issue.
At this stage, my pitiful MRR covers costs, etc but not all the time I put in. Soon as I get to 10k+, at $25/cpm it starts adding up to $1k a month, which is a little less painful to look at. :)