Tantor Audio is having a sale on mystery audiobooks. The mp3 downloads are $6.99. Here is a link to the list:
http://www.tantor.com/BookList.asp?B...tems=10&page=1
The main search page listing often shows the price for another format but if you click on the title for further detail, you'll see the mp3 download at the correct price.
I was surprised to see a couple of urban fantasy series in the mix so even if you're not a hardcore mystery fan, it's worth checking out.