1. A computer-implemented method comprising: a) for each of a plurality of ads, determining, by a computer system including a least one computer, an ad preference value for each of a plurality of ads as a function of performance information associated with the ad and a maximum cost for a result bid associated with the ad, wherein the performance information associated with the ad includes an actual or expected selection rate of the ad; and b) determining, by the computer system, a cost for each of the plurality of ads using performance information of the ad and the maximum cost for a result bid associated with an ad having a next lower ad preference value to the ad, the determined cost being lower than the maximum cost for a result bid associated with the ad.