Most 6" x 9" books, in my experience, use about 350 words per page. However, that isn't good enough. And all the other methods suggested, while they are good for an approximation, won't be accurate enough to create your final cover artwork.
To accurately calculate the spine width you need to know the exact number of pages in the book. At CreateSpace you can then multiply this by the thickness of one sheet, .0025" for their cream paper, for instance, to get the exact spine width.
This is important because the full cover wrap dimensions have to be correct and, at some printers, if you are off by a few thousandths of an inch they will either reject the file or "make it fit" which may not be the result you wanted.
Hope that helps.