I was doing some Google searches for books and I noticed a few sponsored results from Amazon.com. I thought it was interesting that they were buying text ads to link to their site. I checked out the code on the advertisement and realized that some 3rd party was buying Amazon ads and adding an affiliate link to the book. Here is an example:
Affiliate Commission: 4% per $15.00 book = $.60
Google Cost: $.02 per click
You get 5% conversion for every 1,000 clicks which is 50 purchases.
Total Revenue: 50 * $.6 = $30
Total Cost: 1,000 * $.02 = $20
Seems like a simple way to make money to me. As always, a burrito will suffice if you do this idea.