Unfortunately, almost every book trailer that I've seen is so poorly make, it actually makes readers less likely to read the book. People have discovered that they can put a few pictures together with a music track and throw a few words into the mix. The result ends up looking like a cheap Power Point presentation.
Book Trailers Done Right
Components of a Great Book Trailer
Sounds and music can make all the difference. Even the best quality footage will fall short if your music doesn't fit or if your sound is done poorly. Try watching either of those book trailers with the sound muted. I recommend using a professional sound designer like Nate Hoffman at Blufire Studios. His rates are extremely good, especially considering the amazingly high quality of work he does.
A good editor is as essential for your trailer as for your book. Find someone who can take your video footage and piece it into something that reflects your screenwriter's vision and the heart of your book. A good editor can spin straw into gold. Even if you can't afford the best video footage, when it comes to hiring a video editor, don't skimp! A decent editor can work miracles, taking your book trailer from mundane to extraordinary.
Remember that a book trailer isn't just a vanity project, it's a powerful marketing tool that can increase readership and generate sales. Just like a book cover, viewers will see it as a reflection of the quality of the book before they (hopefully) read it. What does your book trailer tell them?
Bottom line, if you're going to do a book trailer, do it right or not at all. Yes, a good book trailer can be expensive, especially if you need special effects, but it should pay for itself.