Just as a personal datapoint. Personally I know that my impulse buy limit for ebooks is up to 15$, over that I hesitate, spend a lot time thinking and so on. For books my impulse buy limit is around 25$ but that's because I can resell it if I don't like it and because while I enjoy reading fiction as ebooks, I prefer paperback for technical content.
Another point where I'm a bit irrational, if I find out that an ebook had a sale for a substantially lower price than now (like the 15$ sale for the "authoring ebooks" book), I'm much less likely to want to buy it at full price because then I feel cheated... It's the same thing for Iphone Apps and so on...
I'm not sure if there are many people like me, it's a bit irrational but it's something that might be a good idea to take into account before doing a sale... By doing a sale you fix the value of your product in the mind of the customer to the price of the sale.
