Audible seems to be running an unadvertised series sale. Books in the sale series are priced at $6.95. I can't find any details, but I have found the following series with the "Series Sale" flag and the sale price. There may be others, so check your wish lists.
I don't have any idea how long this might last or in what regions they might work. Hopefully I am not the only one seeing these.
SCIENCE FICTION/FANTASY
Night Angel Trilogy By Brent Weeks
Sookie Stackhouse by Charlaine Harris
The Takeshi Kovacs Trilogy by Richard K. Morgan (The series link is broken on this one. You either have to search for the books or the series manually.)
Altered Carbon,
Broken Angels, &
Woken Furies
Monster Hunter by Larry Correia
Honor Harrington by David Weber
Baroque Cycle by Neal Stephenson
MYSTERY/THRILLER
Royal Spyness by Rhys Bowen
Mary Russell and Sherlock Holmes by Laurie R. King
Walt Longmire by Craig Johnson
ROMANCE
Huxtable by Mary Balogh
McCabe by Maya Banks
Trap Trilogy by Tracy Ann Warren
TEEN
Kane Chronicles by Rick Riordan
Peter and the Starcatchers by Dave Barry & Ridley Pearson
Iron Fey by Julie Kagawa
Uglies by Scott Westerfield
Morganville Vampires by Rachel Caine
Tiger's Curse by Colleen Houck