Amazon introduced Kindle Match Book, an ebook and print bundling service for authors and publishers. Participating publishers (including self published authors here) can offer a digital version of their book for a greatly reduced price. For self published authors, the prices are $2.99, $1.99, $0.99, or free. For larger publishers, they may be able to negotiate a higher price for the digital version of the kindle match.
Back in 2012 Angry Robot offered digital copies for free with each physical book sale. The result was a tripling of physical book sales.