This may be old news. If so, I apologize for taking up your time reading whatever amount of this post that you do. And please go easy on me. (unlike tubemonkeys, I do have a heart). And, even if it is old news, some newbies may not be "in the know" about it.
I think that I have found a rich source of cheap Whispersync deals. It will still require some digging on our part, but maybe someone has, or we can find, an answer to that also.
If you will follow the path below, you can find find ebooks with "narration for $3.99 or less." Then, if you
sort the hits from low to high, theoretically you will find the less expensive books (Amazon's "low to high" sort doesn't work well for some reason

.
The path is "Kindle Store : Whispersync for Voice: Add Narration for $3.99 or Less". You may have to select "Add Narration for $3.99 or Less" from the menu on the far left side of the webpage.
Let me give you an example of a great Whispersync deal that I found almost immediately:
Flat-Out Love, by Jessie Park. It's the first in a series.
Super ratings at Amazon (I didn't check anywhere else): 4.5 (2,621).
Price of WS deal:
$2.98 ($0.99-ebook + $1.99 Audible book)!
Link to the ebook.