Print magazines and newspapers charge from $5 to $100 (USD) per thousand subscribers depending on the readership — general consumer pubs are at the bottom end while specialty pubs that appeal to wealthy and select audiences such as business owners, gourmet food enthusiasts, or Porsche buyers get to charge more.
Radio and television charge a lot less per thousand, but reach a much broader audience — national television programs in the U.S. can cost a dollar or less per thousand viewers — which can add up to a large dollar amount, but your "reach" or the size of the audience seeing your message can be greater than in any other medium: A hugely popular U.S. TV program can attract 20 million viewers at once.
Web sites are like a mirror of the rest of the publishing world: There are specialized sites, such as AllBusiness.com, which are able to charge a high advertising rate per thousand pageviews, while broader sites like the main page of Yahoo!, which charges a lot less for a less targeted but much larger audience.
A lot of negotiation goes on in ad contracts, but they start from a fixed pricing sheet. This has a disadvantage for search engine results pages, which can't be managed the same way as relatively fixed editorial site pages. Google originally tried charging a flat rate for ads on its results pages — they got way too many advertisers trying to get onto certain highly desirable pages, and nobody on millions of other pages.
Then some genius came up with the ad auction that is now the norm in search keyword marketing. Instead of picking from a fixed price list, you bid in competition with everyone else and pay based on demand for that keyword and its value to you.
So some people are paying a nickel a click on a keyword that's not in much demand, while others are willing to pay fifty bucks a click. So almost every keyword has a price — and a value. It's a great way to sell as much of your inventory as possible at the best price. Clever!