In today's information-overloaded web, the line between being worth my time and being a waste of my time is thinner than you might think.
What you are paying for is the second look.
Even so, you get these benefits:
> An expanded audience.
As a blogger, I think this is your most important asset. Vastly more important than any money you might get from it. An audience is actual power, and besides: It's not who you know--it's who knows you.
> Word of mouth, because of the first point.
Even if this portion of your audience isn't looking at your ads, if they like what you have to say, some of them are bound to tell other people. These are extra referrals you would not have had otherwise.
This is important because growth by word of mouth is naturally compounded. Over a few years, I bet the difference really builds up.
Now, I don't have any numbers to back this up, but my gut says this stuff will end up earning you more AdSense dollars in the long run than trying to maneuver your audience into captivity.