Amazon Exclusivity: What does it buy you?

The answer is, it depends. Mostly, in my experience it depends on your book. If you're self-publishing in the US, your biggest single market will almost certainly be the Kindle store. This is not to say that other markets should be ignored. Barnes & Noble (Nook), Apple  iBooks, Kobo, Google Play, and Smashwords sell plenty of ebooks.

Pricing and Countdown Deals

However, selling a book is not always as simple as slapping a price on it and saying "For Sale." Amazon limits the minimum price for a Kindle book sold via KDP to 99¢. It encourages you to price the book between $2.99 and $9.99 by offering the maximum royalty percentage for books in that range. If you enroll your book in the KDP Select program, it means you cannot sell it in ebook form anywhere except the Kindle store (This doesn't apply to print sales). It also means Amazon offers incentives not otherwise available.  Specifically, you can run Kindle Countdown Deals where for the limited number of days specified, the book is available at a sale price (US or UK only). As opposed to you simply lowering the price yourself (which you could do at any time) the regular price still shows up, but it will be crossed out to show the book is on a limited time sale.

Kindle Unlimited (KU)*

The other big benefit is that your book is available in the Kindle Unlimited subscription program. KU subscribers are Amazon customers who pay a monthly flat fee. This allows them to read any book they want in the KU program. There's a limit to how many they can have at once, but not how many they can read. Amazon pays the author not a flat royalty per book, but a per-page-read fee. The amount is not guaranteed, but lately for the US KU market it has been running about 0.0048¢ per "Kindle edition normalized page"**. It is usually less in the foreign KU markets, but the US is usually the biggest one anyway. 

Genre, length, and readability will affect how much money your book can make in the KU program. My experience is limited since I have had only three books in this program. The enrollment is not a marriage; you don't have to keep the book exclusive forever. When you sign up, it's for a 3-month period and then you can renew again every 3 months, if you wish. If you don't renew, your book is no longer in KU and you can't run price promotions. With Saronna's Gift, which is a science fiction romance, I made enough with KU borrows to make it worthwhile to say in for two full periods, that is, 6 months. With Turnabout, a YA science fiction novel that is in no way a romance, I never made much in the KU program so I let it expire after the first 3-month period. Note that Turnabout is also considerably shorter-- only about 80,000 words, which meant less money per KU reader.

To sum up, I would expect a book that was a page turner, a romance, and fairly long would do very well in KU. And, for what it's worth, here's my advice:

• Always start in KDP Select to see how the book does
• Set a goal for KU borrows (say a third or maybe half of what the royalties are) and drop out when you go below your goal.
• Once you're out of KDP Select, go for every market you can! 

Footnotes:

* I am aware that KDP Select also gets your book into the Kindle Owners Lending Library, but frankly, I don't think this is nearly as big a deal as KU so I didn't spend any time talking about it.

** Amazon calculates pages in its own way, but Alien Bonds was 128,000 words and Amazon said it had 658 KENP pages. That's approximately 195 words per KENP page.