The first thing I'd have to ask myself is:
"Would I pay $40 for a book about someone's personal experiences with herbs and Indian spices?" The answer is No!
The next thing I would do is research books that are similar to mine to see how much they are selling for. One also has to bear in mind how well established and respected the author is in her/his field.
In the U.S. $40 is a lot to spend on any book unless it's a required college text book. Most people aren't going to want to shell out that kind of money for (personal) reading. One man's opinion! :)