How much do you give to your MIL for allowance to take care of your child?

It's weird when we have to pay them. My hubby and I give her the same amount whether with or without the baby. Groceries and all bills are on us. I believe it's an enjoyment for them to take care of their grandchildren and I don't wish to make it a monetary affair. Of course, we do take care of the baby on our own, bring my in-laws out for meals or just an extra $100-200 during festive seasons.