In the classroom it is important to have some sort of rewards system in place, but buying play or fake money can get expensive, and it is something that you will probably have to keep rebutting due to how easily it rips. It's much more personal when you print your own play money, and will last longer if you laminate it. This is something you can use in any grade level and keep year after year if you laminate it.
I call it Star Student Dollars, but you can call it whatever works best for your class. The colors are interesting and much more vibrant than plain colored money, and it will get your students interested in learning about money as well. Keep this file and print it year after year for a memorable rewards system that your students will have fun with!