I usually request a sample of the text to be indexed - for instance, one chapter. This allows me to assess how dense or complex the writing is, and how much time per page is required. At this point I will provide the author with an estimate of the indexing cost.
I charge in the range of $5 to $6 per indexable page. Straightforward texts are on the low end; dense academic texts are on the higher end. Extremely complex texts may require more than $6/page.
Rates for books published in Canada are in Canadian dollars. Indexing of books published in USA or elsewhere are in US dollars.
Once I receive the final copy-edited pdf version of the book, I will provide in writing (usually by email) a firm rate and total fee. If the author accepts this in writing, this serves as our contract, and I proceed with the indexing.
Invoices are sent along with the completed index. Payment is due within 30 days.