I charge $10 less per day so right now that works out to be $22. It's an approximate cost of food and art supplies that I am not supplying to their child. It's a big enough decrease for them to keep child home when they can and a small enough decrease that my wallet doesn't feel it too badly.
I don't get overly complicated in the contract over it either as most parents only use it here and there or with the occasional week or 2 in the summer. I explain it in person to those who would need it over the summer months though. (ie. teachers, EA's, professors etc)