The price to place a banner ad is determined by many circumstances such as the exchange you join, whether you design the banner yourself or hire someone else and how well your banner does.
Some exchanges are completely free. These are usually groups of pages with similar content. Their only purpose is to share information among people with similar taste. They aren’t used for revenue by anyone.
Revenue can be gained in many ways by webmasters who sell banner ad space. Advertisers may have to pay to place an ad depending on if they use an exchange or not. They then pay more based on the number of views and clicks an ad receives. This is why you will see some websites have something that says, “Please support our advertisers.” In supporting their advertiser, you’re supporting them as well.