Audio publishers work on much the same basis as book publishers, in that they will offer contracts, pay advances, and absorb the costs of production. I don't know whether they take unagented submissions or not, but I expect that's easy enough to find out. And may vary by audio publisher anyway.

As an alternative, as mentioned above, you can pay to get your own ACX made. I believe the rule of thumb is $200 per finished hour. It's slightly more than 1 hour per 10K words (again, rule of thumb). My first book, for instance, was 90K words and came in at 9.5 hrs.

Now, as to whether you should...

My sales are approx. 80% audio, and 20% e-book and paperback. I was surprised by this, as I 'grew up' before audio books were a thing. However, talking to my daughter and her friends, it seems they are very mainstream among millennials. I've actually started using, myself. It's a great way to make a treadmill session go by faster, for instance.

The narrator you use has a huge effect on your sales and visibility. My narrator, Ray Porter, also does Peter Clines and several other high-profile authors. He's hugely popular, and many audible customers will buy books based on the narrator just as much as the author. So if you're going to do it at all, this is not the place to cheap out.