Nobody.

The rights to the book are owned by the Bavarian government, and the German government prohibits the production or sale of the book. In 2016, the rights to the book will enter the public domain. In other countries, it varies a little bit. In the US, for example, the publisher Houghton Mifflin owns the copyright, and any profits they make from selling the book go to them.

In sum, the book is essentially royalty free for the simple fact that no one person or entity really owns the rights to Mein Kampf.