Martin Bertram, author of Vanity of Vanities, sent me a link to his trailer. According to Martin, "This was very inexpensive for me and I think it turned out well. I got a grad student in graphics/animation who needed a project for his coursework to do it for $50, plus I paid $150 for the graphics/3D models, $75 for a voice actor to do the 3-minute voice track, and $50 for the royalty-free soundtracks. All in all it cost about $325."
Now I don't think that his book trailer is the best one I've ever seen. And his voice talent was not talented. Indeed, the narration devalued the rest of the trailer. But, for the price, it was a decent trailer. I'll be interested to get a report back from Martin later on how whether or not the trailer helped him to sell any books.