One indicator is to look at best selling and/or popular books in your genre to get an idea for there page length.
Put yourself in the consumers shoes and ask yourself if you were browsing books in a similar genre and both were priced the same, but one seemed to offer more information which would you most likely purchase?
It has been my experience that there are a number of factors that contribute to a book selling or not selling. One of those factors is page count. Other factors include in no particular order your branding, marketing and promotions campaign, you ability to speak in front of people on a regular basis, your media hype machine (publicity), recommendations/endorsements from respected people in your genre, your burning desire, practical written plan and daily execution of that plan.
Visit Self Publishing Your Own Book for more details on publishing, marketing, selling your book. Have you considered publishing your book as an ebook? You may want to read 12 Reasons to Self Publish Your Ebook.
