Adsense publishers may not be aware of this. In certain exceptional cases, Google charges a certain fee from website owners who use Google sitesearch with Adsense.
Update: Google Adsense for Search reports page is now displaying the following information forAdsense publishers:
Revenue from AdSense for search may be offset at the end of the month by costs associated with performing searches.
According to Google Adsense policies, Adsense publishers with very high numbers of searches in relation to their revenue may have an amount deducted from their final AdSense for search earnings.
Google incurs a cost for each search that is performed through AdSense for search, and generally we cover this cost through our portion of the earnings from advertiser clicks.
The AdSense for search fees will never be greater than the publisher's AdSense for search earnings, so no publisher will owe Google at the end of the month as a result of these fees. Earnings from AdSense for content clicks will not be affected. The adjustment will take place at the end of each month, when earnings are calculated.