Setup a bare git repository in your DropBox folder and clone from it into your development folder. The DropBox folder will act as your "remote" origin. Now you can push and pull changes to it. It will physically get backed up when DropBox notices file changes.
http://www.inkmesh.com is another ebook price comparison site. I had the same idea a few months ago, but couldn't really think of any way to improve upon inkmesh and ebookprice. Plus, I've found that Amazon, BN, and Borders usually have the same price on popular books. I think Amazon sets the price and the others match it.
Brewin, thanks for enlightening me on InkMesh.com. That is exactly what I was looking for. I am definitely putting down that idea. How about my other idea?
I created it to learn Ruby, Sinatra, JQuery, and web development in general. It was a fun little project.