I recently competed some work for a client reviewing his books. He wasn't an author, but a third party company who, I suppose, was charged with securing reviews for his roster of authors.
I charged $35.00/$40.00 per review, including the cost of the book, if I wasn't “gifted”; this was for a 180-200 page book.
I'm surprised to see that paying for reviews these days is industry standard. It was always my assumption that people just “go” and review your book. Not the case.
But you can also find industry rates for writers for every kind of service you provide, across every industry imaginable, in writing directories such as “The Writer's Market.”