I see a price on the site link of "$9.95/month for one audio book (Silver Membership)" and about $25/mo unlimited rentals (i.e. as many as you can listen to and get to travel back and forth during a month). Looks very good. But there are inexpensive alternatives if you are lucky enough to live near one... IggleVideo (associated with Giant Eagle grocery stores in the US) and Cracker Barrel restaurants.
I only started listening to audiobooks recently, but now I'm hooked. They are perfect for my long commute every day, especially with all the construction I have to go through. (Yuk!)