Charges for indexing are generally by the indexable page. The cost per page ranges from $1.00 for young children’s non-fiction to $5.00 for the most complex work. I can also quote a price per word, if that works better for a client. An indexable page is any page that the indexer needs to read in order to look for indexable topics. Normally, introductions, prefaces, and other front matter are not indexed.
The final figure is based on the density and complexity of the text (how many entries per page, on average), time available for the indexing (rush jobs are higher), and other factors such as page and font size, and whether more than one index is wanted e.g. author index as well as subject index.
Please contact me to discuss your project and its probable cost. I will be happy to index a representative chapter without charge and then quote a price for the whole book. Some kinds of jobs are better priced by the hour or by the entry. If this is the case, I will quote the price in that way.