What's the confusion here? This is simply where the supply curve and the demand curve meet, and since the supply curve goes flat as soon as the number of copies sold reaches the revenue break-even point, it's all about demand. This ironically is one of the few times when the supply curve is almost backwards; development costs for an e-book are basically fixed.
Some books-as-products will never have a very high public demand but will be essential to those that actually need them, and will command a high price even with low demand, and arguably that low demand will raise the price to pay for the production of the text.
It's expected that many books will see an increase in demand as the price drops, but only a portion of that curve will be a sweet spot, and predicting or measuring it may not help a whole lot. It'd be like trying to quantify the Laffer Curve, it's really not possible without experimentation, and even that experimentation may only be good for a short amount of time or on that single volume.
I feel that e-books are too expensive. I buy a paper copy and I'll have it forever and I can insure it against loss, and it'll always work as there's no medium or machine involved that has to be able to interpret it. E-books are useful from a search perspective, but for leisurely reading they come with baggage that makes me think they should sell for about a quarter of what a dead-tree edition sells for at any given time.