ACX makes it easy; opt for royalty-share and you pay nothing up front, narrator does most of the work. It's true that if the uinderlying ebook doesn't sell well, though, don't expect a lot of audio sales. But sometimes there are surprises. Also, the sooner they are released, while the ebook is still organically charting, the better the audio tends to sell. Mine usually come out a couple months after the ebook and they sell but I have been trying to tighten that time up. Also, I've noticed good audio sales following a BookBub featured deal, regardless of how long the book has been out.