Well, that's a good comparison. There is a mass market which overcompensates lower prices with volume.
Still I see a point that this is not true for all books, uhm, images I meant. There will always be a market for images that will not sell a single license more if you lower the price because the number of potential customers is very low or even one (compare it to scientific or special interest books if you will). For those images you have to find a way to figure out the maximum that this one (or few) customers is willing to pay to maximize your profit. In many cases you will find that the one customer would have no problems paying 50, 100 or 500 Dollar to use that image once.
So while for mass market images it's true to say, you can sell your image 5 times for $100 or 200 times for $5, for some images the equation is you can sell it 5 times for $100 or 5 times for $5. Difficulty, of course, is to find out in advance which images are best for micro and which are suited better for mid and macro.
You make an excellent and thoughtful point. The business of selling books images and books are not exactly alike. Your comparison of limited-demand images to scientific or other special interest books is valid. Such books do command higher prices, sometimes over $100 per book.
However there is a crucial difference between such books and low-demand images: the author of the scientific book may only write one or a very few books in his or her career. But the photographer or illustrator creates thousands of images. So, the reality of the microstock business is that yes I will create a few limited-demand images (and as you say, I will not know which ones those are going to be). But it doesn't really matter. As long as the majority of images which I create do sell well, and a few of them sell very well, I just write-off the limited-demand images. I can still make a good living in micrstosock whether those limited-demand images sell or not.
The limited-demand images don't really matter at all in the long run. And so the pricing of them is irrelevant. What matters are the mass-demand images. They are what the microstock business is about. (If we knew which image were going to be limited-demand, we should perhaps put them in RM, but, as you say, there is no way to know that, only the market knows that.)
