There are many types of media buying resources online. PPC, CPM, CPA, etc so I have provide some information below to help you understand this.
Pay per click (PPC) (also called cost per click) is an Internet advertising model used to direct traffic to websites, where advertisers pay the publisher (typically a website owner) when the ad is clicked. With search engines, advertisers typically bid on keyword phrases relevant to their target market. Content sites commonly charge a fixed price per click rather than use a bidding system. PPC “display” advertisements, also known as “banner” ads, are shown on web sites or search engine results with related content that have agreed to show ads. This approach differs from the “pay per impression” methods used in Facebook, television and newspaper advertising. Similar to the pay per click model which often uses a bidding system, with the online pay per impression method, advertisers bid how much they are willing to spend for their ad to show up 1000 times.
Among PPC providers, Google AdWords, Yahoo! Search Marketing, and Microsoft adCenter used to be the three largest network operators, and all three operate under a bid-based model.
Cost per mille (CPM), also called cost ‰ and cost per thousand (CPT) (in Latin mille means thousand), is a commonly used measurement in advertising. Radio, television, newspaper, magazine, out-of-home advertising, and online advertising can be purchased on the basis of showing the ad to one thousand viewers. It is used in marketing as a benchmark to calculate the relative cost of an advertising campaign or an ad message in a given medium.
The “cost per thousand advertising impressions” metric (CPM) is calculated by dividing the cost of an advertising placement by the number of impressions (expressed in thousands) that it generates. CPM is useful in comparing the relative efficiency of different advertising opportunities or media and in evaluating the costs of overall campaigns.
The American Marketing Association has also defined cost-per-thousand (CPM) as “A simple and widely used method of comparing the cost effectiveness of two or more alternative media vehicles. It is the cost of using the media vehicle to reach 1,000 people or households” [AMA], which is a comparable definition.
For media without countable views, CPM reflects the cost per 1000 estimated views of the ad. This traditional form of measuring advertising cost can also be used in tandem with performance based models such as percentage of sale, or cost per acquisition (CPA).
Here are a few good advertising sources
Microsoft Ad Center – http://adcenter.microsoft.com
7Search – http://www.7search.com
ClickSor – http://clicksor.com
Lead Impact – http://www.leadimpact.com
TLV Media – http://www.tlvmedia.com
LookSmart – http://www.looksmart.com
Media Traffic – http://www.mediatraffic.com
Traffic Vance – http://www.trafficvance.com