I'm looking to take some private classes on cake decorating to learn more than what I currently know, and for the sake of time, I need private classes. How much should I be looking at paying per hour?

the price per hour will depend on MANY things.
1) what do you want covered in the classes, more specilalized techniques cost more.
2) Are you traveling, or are you wanting them to travel. Paying their travel expenses can get expensive quickly
3) WHO do you want to take lessons from? The best in the biz cost more, expecially as they win more awards and get certifications.