I generally try to buy things piecemeal - I'm a believer in Felix Dennis's adage that "Overhead walks on two legs." So I try to buy things individually, which lets me have a good grasp on where money is going.
But Audible is awesome. At some point, I made the switch from reading to listening to most of my books, and the service is bloody amazing.
They have a "credit" system where you sign up, and you get one credit for month, and one credit gets you an audiobook. $7.49 for the first three months, then $15 per month after that. They also send out targeted offers to buy credits cheaply.
I just finished Anthony Everitt's "Augustus" and got started on Ted Turner's "Call Me Ted." Just bought Jan Morris's Heaven's Command from the Pax Britannica series too. They have tons of fiction if that's your thing, but I'm more impressed with their catalog of history and biographies which are the hardest kind of books for me to find digital copies of. I'll pick up Ron Chernow's "Washington" and "Hamilton" biographies after I finish Call Me Ted and Heaven's Command.
Also, another option to think about - a friend of mine who is huge into audiobooks shared a tip - you can share an account with up to 3 mobile devices, so my buddy splits a 2-credit-per-month account with his two roommates and they can all listen. It's a cool option if you have a regular reading club.