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Let's say you have a google search box on your page and the users types in 'widgets'. He get's some results on 'widgets' and one ad on witch he clicks... bling bling!
He wants to refine his search and types in the little box on top of the page 'blue widgets'... same scenario... bling bling!
'small blue widgets' bling bling!
You get the point...
in contrary with a content ad, the user will show itself multiple ad's... a user normally only looks once at a page... so also only once at your content ad's... get the picture?
I find that search clicks seem to be in a narrow price band, usually 10 to 50 cents per click. It never seems to get the 3 cent clicks, and never gets the $1 clicks. CTR and eCPM is higher on search for me as well.
But most people that come to the sites never use the search feature, so the number that counts comes out in favor of Content.
Still, if you feel that you need to provide internal search for those who do want it, AdSense for Search has the advantages of being free, familiar to visitors who use Google for Web search, and a revenue source instead of an expense.