If you're looking for something lower cost you might want to hold off for a couple of weeks. Most of the 3rd party suppliers won't be selling a cover for the K3 until after release of the K3 because they need to get a K3 in their hands in order to design the covers. From what I understand the K2 covers aren't going to work well for the K3 because the of the change in dimension.
Until then, a padded mailing envelope works great at keeping a Kindle safe in your purse.