Makes sense. I've been looking into this as a self-published author, and your numbers definitely work out, or should even be maybe more extreme. Selling an ebook through Amazon for $5 gives a 70% royalty, or $3.50. Print versions through CreateSpace are going to vary depending on production values, but may end up being around $5 for the cost of creation, and then Amazon will cite a minimum price (I was seeing around $10 or $12, again depending on the number of pages), though that doesn't give the author the same royalty. I think I had to pick $12.50 (shorter book) and $14 (longer book) to hit the same $3.50 royalty. So at least from the perspective of a self-published author, the ebook can be less than half.
Now that's for print on demand. A traditional publisher run definitely costs less per physical book, so the ratio of electronic to print price ought to be smaller. A 30% discount on electronic may be reasonable.