I've made my share of book trailers so I thought I'd share some tips on how you can make a trailer without spending a fortune. It's also fairly simple, so even if you aren't computer savvy, you can figure it out pretty quickly.
First, your computer comes with programs to create movies. I use iMovie because I have a Mac. PCs usually have Movie Maker. If you don't see an icon on your desktop, search your Applications. I'm willing to bet you'll find a program. The programs are designed to let you drop pictures into the frames and then add text, voiceover, music, etc. You get to set the duration of each clip, which is great for adjusting the length of your trailer to fit your music. There are even transitions to choose from between the slides.
Once you locate your program, you need to find images for your trailer. A very cost efficient way to do this is to create a book trailer for an entire series instead of each book. You can then use your book covers and crop them for images. That's what I did for the Touch of Death series. All the images, with the exception of one, came from the front and back covers of the books.
I did the same thing for my Campus Crush series, written under my pen name.
If you have a budget that allows you to purchase images for your trailer, I suggest royalty-free stock photos. I use Shutterstock, which allows you to purchase images on demand. I usually opt for the package of five downloads, which isn't expensive at all, and you can almost always find a coupon code online to take about $10 off the price.
For The Monster Within, I opted to purchase images instead of sticking to my cover. I did find some sites (thanks to an article in the SCBWI Bulletin) that offer completely free images as long as you credit them. I found similar sites for the music tracks. Both are great options for keeping costs down. The Monster Within Book Trailer uses paid images and free images along with free music. I made sure to credit my sources on the final slide.
Another option is to film your book trailer yourself and hire some actors to portray your characters. I haven't tried this myself just yet, but maybe one day.
I recommend playing around with whatever movie program your computer has and get used to the different features and how they work. You'll be surprised by how easy it actually is to create a trailer for your book.