Last month one of my niche sites made it to the front page of reddit (in the TIL subreddit). Thought it was interesting that I got a huge influx of traffic (20k visitors), but only an additional $12 in ad revenue.<p>Lesson learned: Not all traffic is created equal!
Did you adjust for time spent on site and/or bounce rates?<p>I've found this kind of temporary traffic burst from high-profile sites is often very transient traffic, people clicking on a link out of curiosity because they saw it somewhere, with a very high bounce rate. It's the kind of traffic from people who open 30 tabs and then briefly visit each one. Doesn't matter (at least for my sites) whether it's Reddit, Slashdot, HN, an NYT Blog, etc.—almost never has the same CPM as "regular" traffic, regardless of the burst source. Visitors who come via organic search or bookmarks are much more likely to spend more than 1 second on the page and read/interact with something.
That implies about $1-2/CPM. Which is not an unusual value.<p>20k visitors for the front page of Reddit sounds low, though. I've hit the front page (with <a href="http://www.reddit.com/r/dataisbeautiful/comments/2ac8ba/who_performs_the_best_in_online_classes_by_age/" rel="nofollow">http://www.reddit.com/r/dataisbeautiful/comments/2ac8ba/who_...</a> , also a default subreddit) and I received 150k visitors.
Getting on the front page of these communities is great but I wouldn't solely look at them as a source of customers. Sometimes you have to create awareness before you have enough credibility for people to sign up. This is especially true for developers. Devs are more difficult to sell.
Here's the monthly report that has my detailed numbers, if you're curious: <a href="http://www.sideprojectprofit.com/july-2014-profit-report/" rel="nofollow">http://www.sideprojectprofit.com/july-2014-profit-report/</a>