A professional Indexer usually charges by the "indexable page". Costs can also be calculated by the hour, or per index entry. INDEX-S rates hover between $3.35 to $4.25 per indexable page (6-10 items per page). Rates depend on the depth and complexity of the text. A firm estimate can be projected based on the provision of sample chapters from the book to be indexed.
MORE INFO ABOUT FEE STRUCTURES AND INDEXABLE PAGES (excerpt from backwordsindexing.com) by Martha Osgood
Although the indexer will need all of the book's pages, the "indexable page" does not usually include the front matter (title page, table of contents, preface, introduction, dedication, etc.) unless those pages are to be indexed. Nor does the term "indexable page" include the back matter, again unless it is to be indexed. It does include short pages, figures, graphs, and pictures.
Even the indexable page method is open to adjustments based on the needs of the book. The actual cost of an index can vary, based upon any of the following:
• many names, especially foreign names with diacritics - each name needs to be spell-checked and verified with the bibliography by hand and each author name may need to be connected with the full title of a work
• the number of columns per page, the size of the print, the margins, and the size of the page
• multiple indexes for one book - scripture, author, subject, Latin names / familiar names, legal cases, tables, bibliographical index, etc.
• book density - the average book has 6-8 indexable entries from each page
• specialization and difficulty of subject - medical, legal, philosophy
• restrictions on the length of the index
• amount of time allotted or changed for the indexing process
• delivery of manuscript by chapter (rather than all at one time)
• re-indexing due to pagination changes after the indexing process has begun
• multi-part locators - page references including volume and page numbers, or page and section numbers
• specificity - the extent to which a concept or topic is identified by a precise term on the hierarchy of genus-species relations
• exhaustivity - the extent to which concepts and topics are made retrievable by means of a quantity of index entries and synonyms
• depth - the degree to which a topic is represented in detail in an index (#s of precise terms, levels, cross-references)
• mixed sorting orders - biographies may need hand-sorted entries in chronological rather than alphabetical or page number order