I always enjoy stopping by Janet Reid’s blog. She recently posted an answer to a question regarding paid book reviews. While I could paraphrase it, I’ll let you read her own words. http://jetreidliterary.blogspot.com/2016/01/rant-paid-book-reviews.html
I agree with her 100%. There appears to be a whole cottage industry designed to separate gullible writers from their money. Writers should spend money on editing, cover design, typesetting and eBook formatting, but never on book reviews. I would also avoid, as Ms. Reid mentions, anything that bundles marketing with a book review; you’ve heard the saying don’t eat where you crap? If you want an easy, relatively speaking, way to get honest reviews, sign up for a Goodreads author account and setup a give-a-way. Sadly you will not get 100% of the people providing a review but you will get many and they will be honest.