I use one of two methods for pricing, depending on your needs.
Per indexable page
Most common is per indexable page. Indexable pages do not usually include front matter (for example, the foreword, the acknowledgements) or back matter (for example, the bibliograpy), unless it is requested. It does generally include charts, graphs, tables, photographs, illustrations, and partial pages. The per page rate is applicable when the book is in final form and there will be no changes to the pagination. The cost per page depends on the density of the text (average number of entries per page), the complexity of the text, the subject matter, your specific requirements, and required turnaround time. Whether you need a single index or separate indexes for author, subject, etc. will be considered in the quote. My per page rate ranges from $3.00 to $6.00.
Sometimes an hourly rate makes more sense. For example, if the index is to be embedded, the book is not yet in its final form when the index is created, so a per page rate would be difficult to determine. If there are pagination or manuscript changes in a book that has been indexed at a per page rate, the necessary index changes will be done at an hourly rate. My hourly rate is $40.
Before the quote is sent, I will need you to send two sample chapters (not the first or last chapter) for evaluation, an approximation of the number of pages to be indexed, the desired size of the completed index, start and deadline dates, and any other information you think would affect the production of the index.