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That is where the majority of your advertising expense at Overture will originate.
Before Overture's partnerships evolved, "Goto.com", was fairly obscure in the "overall" scheme of things.
different engines have different degrees of "security" to prevent them from billing you for "questionable" clicks. we've had problems with some engines, and no problems with others. i wont get into names, in the interest of diplomacy, but suffice it to say that the most successful engines are such because they instill confidence in their advertisers (and have good reach).
there's a whole host of possible abuse scenarios out there - some more complex than others. many can be prevented with diligence, but the best ones can sneak by even the most solid "detection" system.
so keep in mind that the bottom line for you, in terms of determining whether or not a campaign is "worth it", is to determine what you "get" from that campaign beyond just clicks. if there's a little fraud, but you still make sales and a profit, then its still "worth it" for you (it just means your actual cost per visitor is higher than your obvious cost per "click")...
everyone has their own yardstick by which they must measure.