For the sake of argument, let's say there are 1M books. Average sales 500 a year. That's just over 40 books a month or 1-2 sales a day. That would put the book at what rank? 50,000? 30,000? Does anyone know? Let's say it's 50,000. That means that 5% of the books have 50% of the sales. Is that right? I'm not good at math.
As a side note, I just noticed this interesting breakdown for review ratings...
Avg. Customer Review
* 4 Stars & Up& Up (220,403)
* 3 Stars & Up& Up (256,431)
* 2 Stars & Up& Up (264,457)
* 1 Star & Up& Up (272,751)