Well keep in mind that the estimated bids that you see advertisers are spending for that keyword is about twice as what YOU will paid if someone clicks your adsense ad for that keyword.
Example; The advertiser Rats-R-Us pays Google $2.30 CPC for the keyword "horny rats" in their adwords ad. You set up an arbi site based around "horny rats" and display adsense ads related to it. The ads from Rats-R-Us show up, and visitors click on it. Now... you will NOT get $2.30 from that click, but more like 20-50% of that amount. Remember that Google also gets their share of that click, and although they have never stated officially what their percentage is, it is assumed to be at least 50%.
What also can lower the amount further is smartpricing and other factors affecting the current cpc.