1. A method comprising: receiving a request from an advertiser of an online system identifying an advertisement for presentation to one or more users of the online system; retrieving information associated with the advertisement; determining a plurality of candidate budgets for the advertisement based at least in part on the retrieved information associated with the advertisement, each candidate budget specifying an amount of compensation to be provided to the online system for presenting the advertisement to users of the online system; determining a likelihood of a purchase of the advertisement at each of the plurality of candidate budgets based at least in part on the retrieved information associated with the advertisement; determining an ad purchase interface comprising a set of budget options, the budget options selected from the plurality of candidate budgets based at least in part on the determined likelihoods of the purchase of the advertisement at each of the plurality of candidate budgets; and providing the ad purchase interface for display to the advertiser.