Auction prices for a quantity of items, the quantity of which is not determined prior to auction, are equilibrated among bidders to reflect supply and demand for the items. For example, in auctioning Internet advertising opportunities, the first available opportunities are auctioned to a high bidder. Later in the course of the auction period, once the high bidder's budget has been exhausted, the price for advertising opportunities is reduced. The difference in price paid by the high bidder and the reduced price for the quantity of advertising opportunities purchased is credited to the high bidder's account. Once the high bidder's account regains a level sufficient to acquire additional advertising opportunities, the high bidder may continue to acquire advertising opportunities. Other bidders similarly are credited the difference between prices previously paid and the current price. An equilibrium price is reached based on the recursive adjustment of prices paid by the bidders.