This is an excellent post by Author Nicholas Rossis on return on investment for paid advertising options. Make sure you read the details. This data was sent in by folks running discounts on their books (a sale) . Results also vary due to how well your ad is designed! For this author to gather this info and share it is extremely generous and helpful. Please support his work by turning in results you have had.
There is a helpful graphic and also lots of links to follow up on. This is up-to-date, current information! Here is the link to his great post.