This forum gets decent traffic; but I really don't make much on adsense.... Why? Well it's mainly blogging discussion and they typically aren't high paying keywords. Furthermore it's almost exclusively WEBMASTERS who come here. We all know what ads look like and typically never click.
I sold a Run of Site advertising campaign to someone on a business blog of mine and removed adsense. About a week ago I decided to add adsense again. Sure enough it made $3.11 yesterday alone. Needless to say I've started posting again.
What I'm getting at is certain types of blogs generate a higher price per click.
https://adwords.google.com/select/KeywordToolExternal
That tool is your friend; it will tell you what niches are worth entering which aren't. Might I add if you love a niche go for it but if your just looking for something to rack in some $$$ go for where the $$$ is.
Now a word of caution; don't think your the first person to "go where the money is"; with higher price per click you have more competition. I just got lucky I guess as I like home business discussion.