Well with SEO if you generate 50,000 impressions per day with a click through rate of just 5% you can generate $625 per day with an average earning of only .25 per click (
source ).
50k impressions seems like a lot but if they're driving traffic from search engines (and think about how many people play WoW and look for addons every day) and then from the main TTH site (this is just using WoA as an example) 50k impressions isn't that hard to generate especially if you count ad impressions from both sites. 2.5k uniquie visitors with 10 page views each per site per day would do it.
Getting ads on your site isn't that hard but driving the traffic is a little trickier. However WoW addons are a nice easy way to get traffic if you have enough of them and people know the addons you have. I can't imagine the number of people that just google search DBM or Recount on a daily basis but at least a small percentage of those almost have to end up at WoA and then a percentage of those will undoubtedly click an ad.
The income can add up fairly quickly as long as you're getting page views, and TTH has been around a while and has a loyal following.