Same here - they're one of the mainstays of our stock. You can get them on MP3 now too.
There are several reasons for the high prices. For a start, popular as they are, they still don't sell in great numbers, so the markup will be higher. Also, royalties have to be paid to the reader as well as the author, and then there are studio costs. There's also a higher production cost to them. WIth a book, you're producing one item. With an audiobook you have to produce several CDs, case, wallets and cover, and pay for the resources to put them all together - transport, man hours etc.
Do the right thing - support your local library! ;-)