Back when I was among the Top Amazon Reviewers, I'd regularly be asked to review products and (especially) books (primarily those that were self published). I rarely said yes, but when I did I treated them exactly the same as I would for a product I'd bought myself. That included 1- and 2-star reviews... though usually I tried to weed out the crap before saying Yes to such an offer.
The cost really never influenced me. I can afford a $10 novel that I'm interested in. So if you give me that book for free, it's no big deal.
In point of fact I learned to say No to most such offers. Usually novels are self-published only when the author cannot find a traditional publisher. And the traditional publishers usually refuse the book for a good reason. I don't want to waste my time reading crap.... not when there are so many good books to read instead.
Also, the self-published authors really didn't like it when I gave their books less than 5 stars, and magically the review would be downvoted -- as well as the last three unrelated items I'd reviewed. Presumably those neginators were from the author's friends and family. So I just said No, and moved on to books from authors I was sure I'd like.