I have a filmmaker friend who can make stuff look AMAZING, so I enlisted him and his students to make me a book trailer. I pitched EW cold, and I lucked out and they liked it enough to post it (it's still there if you want to check it out).
I had a sales bump from that, for which I'm very grateful, but it didn't make a huge difference in the fortunes of the book. On the plus side, I did not pay a lot for the trailer (under $500), and it represents my book beautifully, and I'm happy to have it in a place where it can get eyeballs. It's more a short film than a conventional trailer, so I feel like it's a companion work to the book, not just an ad.
I didn't get quotes from video production companies, but I would guess most start at $1000 for something like this. I've heard of people paying five figures for video work and not getting the best quality in return. You need to look very, very carefully at people's past work and know exactly whom you're dealing with. A lot of people have nice equipment, can hire actors, etc., but they may not know how to edit their footage into something watchable. Having a sense of exactly what you want is vital.
Overall, I would say EMaree is right; a trailer is not a great promotional investment. I'm glad I did it, but I would not advise anyone to sink a lot of money into it. If you do choose to do it, I would say to use a simple, easily graspable concept and not try to involve a ton of special effects or try to approximate a blockbuster movie trailer. I've never heard of a book taking off because it had a great trailer (not to say this has never happened!), but I have heard of books taking off because they happened to be mentioned by the right YouTuber.