Search Engine(Want more clicks), Advertisers(Want more clicks that turns into buisness) and Users(Good ads with useful inforamtion) all have their own utilities.
All these utilities have to be balanced to make search engine a good adverstising system.
There can be many optimization function for this. One such function can be a simple linear combination expressed as
Utility = a * UtilityAdvertiser + b * UtilityUser + c * UtilitySE
where a + b + c = 1
Here utility for SE may be the cost per click in dollar
for Advertiser it may be the number of converted sales
for user the relevance of ad, whether they got some useful inforamtion
-Bharath
