How Much Would One Invisalign Tray Cost?

I had braces several years ago, and my teeth haven't shifted much as I still wear my retainers. However, my top retainer broke and before I was able to replace it, one of my front teeth shifted slightly. It's not very noticeable to most people, but it bothers me as I would like to have my perfect teeth back! I would think one or two Invisalign treatments should suffice, as it would be a very minor fix. I'm just curious as to a ballpark figure of what one or two trays/retainers would cost?