Very few people get paid to write book reviews these days. In fact the number of print magazines that even print them regularly is declining rapidly, and many genres were never well represented. Print magazines seemed focused on high literature, something that DearAuthor.com thinks might be part of the reason for their decline. So magazines aren't paying for reviews.
Most people now write reviews for online venues which have very limited funds, if any. In fact given that most online venues cost some money to run and earn none, certain suckers who shall remain nameless effectively pay for the honor of writing book reviews. The most your average reviewer is likely to get out of the deal is some free reading. But as people who want to review books are people who like to read books this is probably fair enough. So anyway, websites aren't paying for reviews.
It was therefore probably inevitable that some review venues would try and reverse the revenue stream entirely and try to charge the authors. But as far as I am concerned, no author should ever pay for a book review. A review is useful only to the extent that potential readers will heed it. Potential readers heed sources of information that are useful and impartial--one reader providing advice and recommendations to another (and yes, it is pretty easy to tell when this is no longer the case). This honesty and impartiality rather falls apart if the reviewer is taking money from the publisher or the author.
Besides, readers don't run out and buy books just because there is a review somewhere on the endless wasteless of the Internet. Very few sites have the influence to sell many books and they (with a few lamentable exceptions) do not indulge in profiteering. For the rest, avoiding a month or two delay, or even getting a review versus not getting one simply isn't worth the $10-80 they are charging. Those people that trade on their reputation to charge for reviews frankly don't have much of a reputation left. And none of these usually glowing fee-charging reviews is going to do what the author wants, sell books.
What sells books is honest reviews from influential sources. Or maybe even a few copies from less influential sources so long as the 'cost' side of the cost/benefit ratio is keep to a minimum. Readers listen to other readers who write reviews out of a enthusiasm for the genre. They listen to reviewers who read the book because they wanted to read the book.
Ergo, the more you pay, the less the review is worth. It is one of those rare cases where the best things in life are indeed free.
[Edited to Add: interesting blog post on this issue by Monica Jackson.]