It's happened to me innumerable times that I leave something in my Amazon shopping cart, and when I come back to it the price has changed. Sometimes it went up, sometimes it went down. The only thing it seems to do consistantly is change.
At Christmas I noticed this more than ever. Toys that were $30 became $5 and back up to $30 again. Unfortunately, I didn't find this until now:
Refund Please - Data Entry
You tell the site when you bought the product and what you paid, and they send you an email when the price drops. You can contact Amazon and they will refund the difference.