The fee for indexing a book will depend on many factors, such as the density and complexity of the text, whether there are non-textual items to be indexed and the time available to complete the index. My rates are usually between £2.80 and £3.50 per printed page, or £9.00 - £9.50 per thousand words of indexable text (excluding bibliographic and other non-indexable material).
The Society of Indexers guide to indexing fees may be useful. It also describes other issues that are involved in negotiating rates.
If you have a book that needs indexing please contact me to negotiate terms.