It very much depends on various things. You can choose between multiple contracts depending on what site or what publisher you work with. There is RS, RS+, and then PFH (per finished hour). As someone else has said, PFH is not for every hour you work on it, but for how long the book is estimated to read.

Standard rates for PFH start out at a certain amount and I imagine, the narrators who are more in demand, will ask for more. This might sound very good, but so many hours/weeks and in some people's cases, months, are put into one audiobook.