Not the SDMB, but others.
By the day?
By the number of hits?
What?
there are as many answers as there are websites. Contact the site in question for how they handle it.
Many small websitrs don’t handle their ad sales themselves. Instead, they have a standing contractual relationship with an external service provider (Google is big in this business) under which the ad to be displayed to a particular visitor on the site is supplied by the provider, selected on the basis of a profile of the visitor’s browsing and only shopping habits. The ad revenue is shared among the two, but I assume the pricing is done by the external provider.