I worked and taught stamping classes at my LSS for 3 years, just quit about a year ago. All stores treat their instructors differently, so you never know until you talk with them. Mine used mostly employees to teach so we were paid our hourly wage for prep time and class time, so you got paid no matter how many people came to class, and the store paid for all supplies used in class. (As employees we also got a huge discount on personal purchases)
Alot of stores want outside instructors to pay for their own supplies and make a percentage based on how many take the class, which can really cause the instructor to loose out on time and product if you don't get a good showing in class.
Be sure to ask lots of questions and get all the info before you decide to teach
Good luck!