Thing is, getting someone to agree to a review is a crapshoot. Even if you've managed to ink out the best book of the year, there'll be plenty of folks who don't like it. Trust me on this. Say your reviewer is a woman who was once jilted by a guy who just happens to have the same name as your lead character, lost a child the exact age as one who dies in your story, or has a fight with the boyfriend or the boss the day she sits down to read your novels. Maybe yours is the hundredth mystery/romance/science fiction/western (take your pick) she's reviewed this year and she's simply had it. Makes herself a big old martini, sits down at the keyboard and takes her snarly mood out on little-old-you.
Paying for reviews is a whole other ball of wax. Some of the most prestigious review sites charge big bucks for a review they promise will be completely honest, which doesn't necessarily mean "good" (see above). That means you can pay a lot of money for the chance to get majorly, where-did-I-hide-those-razorblades depressed. These companies charge a ton for their reviews because they know that you know that if by some miracle you manage to eek out a good one, you've got a leg up in the game.
All of this by way of explaining, perhaps, that I'm fully aware the absolutely stunning review I just received from Kirkus for Matinicus (due out in May) http://www.kirkusreviews.com/book-reviews/darcy-scott/matinicus/, had as much to do with the luck of the draw as anything else. Don't get me wrong. I'm really proud of the book, I busted my ass on it, and I'm ever-so-grateful that whoever reviewed it liked it almost as much as I do.