I had to pay my mil to look after our son. She would write down all the days, hours and cost on a whiteboard and place it where it could be seen from the front door. I didn't mind paying some money, but eventually he went to creche which was on site where I worked.
On the other side of the coin, I have taken my own mother to hospital appointments, family weddings, fetched and taken her home at Christmas (it's a 50 mile round trip along difficult roads) days out and any other times she wanted to avail herself of a weekend break at our house. Not once has she ever offered to pay for the petrol, contributed to a family meal or even appreciated my efforts.
If you feel unhappy and resentful, then choosing an alternative is the way to go. You may find, however, that you prefer your mil to continue if things don't work out. I wonder though, if you were to say to them that you cannot afford the fees they want, would they withdraw their 'services'?
It's worth a discussion - not a confrontation - where ALL of you can put forward your thoughts so that everyone knows about any difficulties. I don't suggest a formal meeting, try to keep it friendly over an evening meal or drink. You are adults after all and no one needs to feel hurt if the discussion is kept open and honest.
Hey ho - families are funny things!