I wonder if a simple and logical metric would be to discount whatever you paid for the ebook from the purchase price of the hardcover.

Offer not good for paperbacks, only because paperback and ebook prices are often (but not always) similar and that would create a likely scenario of loss for the physical producers.

Goodness knows I've bought several physical CDs off of Amazon because not only was the physical CD price less than the MP3 download price for some reason, it came with a free MP3 download of the album instantly. This seems ridiculous to me, but I'll take it.