I recently met such case as my book is pre-order type. When the book is ready, they send me a mail and I check the price of the book which is lower than the price I bought before. Since I wait for several weeks before this book ready, and I feel it unfair that the price now is lower. So I send them letter and they cancel my previous order. And I re-order again at a lower price.
yes, sometime book fair is cheaper. I ordered inferno as soon as the paperback is release, at that time it was cheaper than the bookstores in Hong Kong. But before I got anychance to read it, commercial press is on sale with HSBC card and the price is lower than the price I paid.