Love Disney as well...at 500 pages, if it's as good or better than the Birnbaum guides or anything else like it, I'd say $3.99 is far too cheap. If it's more of a story, as in a literal diary of your experiences and not organized in a way that I could flip to a specific section to find insight, then maybe the $3.99-$5.99 price range would serve better.
At first I thought, as an avid WDW traveler, that I wouldn't be interested in reading about someone else's trip. After all, I've already been there...so unless you have weaved a compelling real-life story, I have my own experiences to re-live. However, your summary made me interested in reading it based on your extensive time invested into your Disney trips. My own opinion is that $3.99 is still probably too cheap, and $5.99 or higher may be worth trying. You have 13 years of experiences packed into a large book...if someone is interested in reading about that, I don't think the price will stop them from wanting to dive in. The culture of "Disney lovers" is an rabid one, where price (obviously) has not prevented them from buying "more" things Disney.

That's just my 2 cents.
Really cool to see such a different type of book on the Kindleboards. I hope if you do go with the higher price tag it works out for you!