Optimal Price of e-books

Interesting interview over at Gallycat with David Carnoy who went the self-published to published route. He says the optimal price for a book is $3.99. Low enough to appear like a significant discount for the traditional books (which are now often well above the $9.99 price point Amazon was aiming for), but high enough that the books appears like it has value. The $3.99 also is high enough to earn the 70% rolaty rate. Authors who self-publish will earn $2.79 for each copy sold.