Google ads works like this:
For every refresh your page gets, it gives you a teeny tiny amount.
However, if people actually CLICK your ads, it's a whole lot more. Now, this is all relatively speaking, since I mean like 60 bucks a month, AT MOST, if you have shittons of people clicking your ads.
We installed it in a forum I moderate a year or two back, and it was largely successful because of our large userbase committed to "helping out"... so much so, in fact, that the AdSense account was suspended and we were left high and dry.