Mother in law

My mother in law is coming in town she said she wants to help us save money because we spend a lot money on daycare and she wants to spend time with our son and spend time with me because I’m Prego and will be having my baby in May but she wants us to pay her to watch our son. How much should we pay her? We didn’t offer to pay her she chargers her daughter to watch her kids I believe she pays her $200 a month.