There are a few versions up on Project Gutenberg, but they seem to mostly be in Middle English. This is one of those books where people have had to translate and edit it to make it more easily readable by today's standards, which means anything done with it in the past 80 or so years is copyrighted.
EDIT: This looks
like the best price you'll find on a reasonably clean, well-formatted annotated copy (I'd trust a Penguin, Norton, or Bantam edition any day over some no-name 99¢ [or free] copy). After conversion fees, it should be about $5.50. Seeing as how you've been looking for a version of it for over a year, it might be worth taking a chance on the mobi or lit version there.