Cost per action is an online advertisement model. In CPA the advertiser pays for each specified action that is linked with the advertisement this includes purchases, form submissions, etc.

The paid action of cost per click advertising can go as low as one cent. This is offered by Clicksor. Other companies that offer similar services, but with higher fees.

How much internet advertising costs depends on which pricing model you go with. These vary from CPA (Cost Per Action), CPM (Cost Per Mille), CPC (Cost per Click) and a fixed flat rate price. Individual clicks on an ad can cost as little as $0.01, whilst monthly or bulk impressions can run into the thousands of dollars.

CPA = Cost Per Action. CPA networks pay commission based on a qualifying action for example: filling a form, providing zip code.

The opportunity cost of doing action A is not doing action B.

So, the opportunity cost theoretically depends upon how much you value action A over action B (ie, value of action A - value of action B).