In some lessons, I reference a token economy. In most classes that I have worked in, I have printed my own money that students can spend on prizes and privileges in my classroom "store" as part of PBIS (positive behavior intervention systems).
To keep things affordable, I generally buy cheap candy (on sale or smartees, tootsie rolls, dum dums, etc.) and buy pizza if students choose to purchase a paid lunch with me. The attached store is one example that I have used in the past.
I reward a lot in the beginning of the year to reinforce expectations, and for more targeted positive behavior as the year progresses. Believe it or not, this has worked at the middle and high school levels (grades 5-12), and my Loot Bucks have been praised as being "like gold - rare, coveted, and worth it" (student words).