If folks didn't already know, Amazon runs a Kindle daily deal. You sign up for the notification and get an email every day about a handful of ebooks on sale. I've scored a few nice hits on that recently, including the Cold Equations trilogy by Dave Mack and Tim Zahn's classic Star Wars novel Heir to the Empire.
Amazon also has a rotating collection of ebooks on deep discount every month. Sometimes you can get some really good books for under $3.