Basically, if you want a bookseller to buy your book, you need to give them a discount that will allow them to make a profit on resale. Big booksellers like Borders for example, can command discount up to 70% of the price of the book. This means that the publisher only makes 30% of the price from selling to them but because they can buy and sell such large amounts, they have to accept these rates.
This makes it very hard for small publishers because they can't necessarily afford to give such large discounts. Booksellers also want more security when dealing with small or self-publishers because they have less of a marketing budget which affects sales and Booksellers don't want to end up with a bunch of stock taking up shelf and warehouse space and isn't selling.
How this factors in to the price of the book: basically, take the cost of producing the book, this is the very minimum money you have to make in other not to make a loss. Then slap on the highest price you think you can sell this book at (look at the price of comparable titles on shelves, Amazon etc and by comparable I mean not just content, you want to look at the size of the book, the quality of the paper, hardback vs. paperback and size of the publisher). Then you have to decide how much of a percentage of this price can you afford to give up.
ETA: also, since you're posting this in the Self-pub, POD section, I assume you're self-published. This makes it even harder because a lot of stores won't carry self-publishers at all, regardless of what discount or returns policy you offer them (basically, if they can't sell it, you refund them their money). If it's a very local book, they might stock a few copies. Booksellers are predators.