ACX has pretty strict guidelines...
It's a pretty good system.
Yes. I was terrified of getting ripped off too. But here is how it works. (At the risk of repeating what others have said.) You only pay for finished hours. ACX estimates how many finished hours based on word count. So on my 80,000 word book, they estimated 8.5 hours. (And it worked out to almost that exactly.)
You can put your book up for auditions and some narrators will record a small sample so you can pick something you like. (You can also pick narrators and try to contact them directly, but I liked the idea of people who wanted to do my book.) I put my book up for audition with a price range of 100-200 for finished hours (no revenue sharing) and almost instantly got several auditions. One voice I really loved. (Tess Irondale, she's great!) I negotiated the deal through ACX for $200 a finished hour. (Which seemed pretty fair given it takes hours to create each hour.)
Now, here's the key, you don't pay until they are finished and you approve the final audio hours. So it's not easy for them to rip you off. (Also, it doesn't hurt to check to make sure they've done some other audiobooks and have a little reputation.) I suspect they're more terrified you're going to stiff them. Their only protection, as far as I can see, is that they have to sign off to ACX that they got paid after it was all done. Then, and only then, your book will be released. (Probably helps ACX has a monopoly, so there aren't many places writers or narrators can run to if they try to scam each other.)
You do, however, have to be reasonable about asking for changes. At the end of the day, my audio book cost me a little less than $2000 and I'm just thrilled with it. Kind of like producing a little movie. I have no idea if I'll ever make the money back, but it's was a really fun experience.