Brian,
I do subscribe to Amazon Audible, or at least did, it's a bit murky because I get my Audible books along with my Amazon Unlimited subscription. So, I have a rather large library of Audible Books, but I haven't paid a membership fee in about 2 years.
But I listen to 4-10 books a month so I think I can help you with this.
I completely prefer download. I wouldn't want all of those CD's lying around. With my grandkids around they wouldn't be long for this world. Besides that I have them all in a neat library which allows me to access any of those books at any time.
Normal range? Hm, that's a bit hard to say. The prices can vary tremendously. If you are dealing with a recent best seller it is not uncommon to see the books priced at $20 or more, sometimes much more, for a stand alone audio book. If you ask for the price that is seen most often that would be between $3.99 and 5.99 if you also purchase the ebook. I have to admit that well over half of my audio books were free as a part of the Kindle Unlimited offer. I don't believe I've ever paid over $5.99 for an audio book.
Hope this helps.