An online system determines the pricing of an advertisement included in a scrollable advertisement unit including multiple advertisements and a display area presenting one or more advertisements. When an advertisement is included in a scrollable advertisement unit, a price for presentation of the advertisement is determined based on bid amounts of advertisements evaluated for inclusion in the scrollable advertisement unit and a scaling factor. The scaling factor is based on the probability of the advertisement being presented to a user based on the position in the scrollable advertisement unit associated with the advertisement. Advertisement characteristics affecting the likelihood of an advertisement being presented to a user may also be considered in the scaling factor.