What do web sites cost to look at?

Ad Serving is most complex when it is used by an Advertising Network. An advertising network buys ads from many web sites and therefore acts like an advertiser user of Ad Serving. When the network buys ads, it tries to place ads on sites where they work best. However an ad network then sells its aggregated ad inventory to advertisers. When doing this, it uses its Ad Serving software as a web site does. In this case it tries to make the most money by only running the ads from advertisers that pay most.