Provided is a keyword advertising system for extracting search listings in response to a search request. The system comprises a memory for storing a search information database; a bid control module selecting a predetermined number of search listings primarily according to the bid amount and an estimated advertising period; a ranking module generating an ordered search result list; and a computation control module computing the advertising period for the received bids. The ranking module orders the selected search listings in the search result list according to the selected search listings' respective bid amounts and the estimated advertising period and the computation control module estimating advertising costs uses the bid amount of the advertiser and average number of clicks information during a predetermined previous period. The computation control module further calculates the estimated advertising period using deposit from the advertiser and the estimated advertising costs. The search information database stores at least one bid received from advertisers, the at least one bid being associated with a cost per click advertising, the each bid being associated with a keyword, a bid amount and a search listing.