This is not to say that I think the cost of a book is irrelevant, or that it should never be mentioned in a review at all.
Reviews That Mention Price Can Be Helpful
So, while I try to pretend (for the purpose of reviewing) that all of my books came from a magic book fairy at no cost to me, it is a common practice among other reviewers to mention the source of the book. Why do so? It provides us with some what that reader's investment in that book. I do feel that someone who spent hard earned money on a book is going to be inclined to really try to like it, while someone who got a book for free might be more inclined to dismiss it over tiny flaws. Or perhaps not.
What I find more helpful, as a potential reader, is when the reviewer references the exact price they paid. I know that sounds crass, but it's true. When someone says, "It was good, but it wasn't worth $12 that *insert retailer here* is charging." That tells me to hold off, wait to find it used or borrow a copy. On the other hand, "It's flawed, but for $1.99 why not give it a try?", while not a glowing endorsement, does make it sound like I don't have much to lose. I'll take a risk for $2.
Price Influences Our Book Buying Decisions
Obviously. How could it not? Unless you're one of those rare lucky ducks with an unlimited book buying budget, you're operating under a weekly or monthly or yearly limit. You want to get the most bang for your buck. For me, this becomes a curious game of mathematical gymnastics. If I have $20 to spend, and I buy 20 $1 books, the law of averages states that at least on of those books will make me smile. But then I've got 19 really frustrating or mind numbing books--which will make hilarious reviews, but will eventually drive me to drink. If I spend a bit more to buy books from mainstream authors that I know and like, I'm less likely to be frustrated by any of them, but my $20 is only going to get me 2, 3 books max. And what if one of them does suck?
The higher the price, the more assurance of awesomeness I need before I'll take the plunge. For a series or author that I dearly love, one which has given me multiple positive experiences and no negative ones, I'll spend my entire monthly budget to buy a hardcover. Not rational, I know, but I want it that badly. A less proven author might get $8 from me, but only if there's nothing really exciting to buy or preorder that month. Same goes with really well reviewed authors that I don't have personal experience with yet. Then there are the unknowns with few or mixed or no reviews. For those, I'm not willing to spend more than $4. And again, it really all depends on how many big releases there are in any given month.
The Issue of Ebook Price
I love my Kindle dearly, I really do. However, there's a special magic to owning physical books. To be honest, I usually only buy ebooks when they're cheaper, or when I'm traveling and need the portability of the Kindle.
I firmly believe that ebooks should always be cheaper. I'm not the first to say so, and I won't be the last. It's a fact that I can't lend, sell, trade, or display an ebook as I do with other books, so the price should reflect that. As in, the price should never, ever be more than $12, regardless of what the print book costs. Ever. I actually find it really insulting when publishers make the decision to put a hardcover price on an ebook.