A cost maintenance scheme useful with an advertising system which orders ads in a manner that maximizes both their relevance and their economic value, and which provides a fair bidding process in which a winning bidder is assured of not having paid too much. Ordering may be done based on accepted maximum ad bid information and/or ad performance information. For example, this information may be used to determine a position value. Cost may be determined based on accepted maximum ad bid information and/or performance information. Billing the determined cost may be subject to a condition precedent.