What is an index?
The index is a roadmap for the reader so that they can find the information they need. The indexer is the link between the author and the reader, to make sure the reader can find the information they are looking for quickly, and point them to any other information they might find useful.
Why use a professional indexer?
The construction of an index takes training and practice. A good index is one which makes the content of the book easily accessible to the reader, containing references to the concepts which the reader wants to find. The author of the work may be too close to the text to be objective about the terms used.
 Can't computers generate the index automatically?
Computer software can generate an alphabetical list of words (a concordance) from the text but it cannot recognise themes or concepts, or highlight related terms. A computer search cannot identify text which has the same meaning but which uses different words. It also cannot differentiate between important references and passing mentions. See Article: Human or Computer Produced Indexes?
How much will it cost?
Rates vary depending on the job but they are based on those recommended by AFEPI Ireland and the Society of Indexers. Before being asked to agree a fee, I would need to see the proofs, or at least a few specimen pages, in order to make a fair assessment of what is involved. If time is too short for that, at least the following details should be given: subject, length and format of book, readership aimed at,  pages/number of lines allowed for index and time available to produce the index.