Fees are agreed at the start so you know what you will be paying – as long as timings are as agreed and there are no changes to the text or pages after I have started work.
My quote will depend upon complexity of the text, the inclusion of illustrations, and tables to be indexed, whether separate indexes are required (e.g. botanical names and a general index), and the time available (mine and yours).
You will rightly want a quality index; you will possibly want it soon, and you will probably want it cheap. There will inevitably be a trade off between these requirements, but I have high standards and will always provide an index I would be proud to see in print. I will not take on work unless I am confident of being able to deliver on time but depending on my other commitments there may be some flexibility on price.