I commend you for offering. That is something alot of people wouldnt even think to do.
As for the cost. When I had two in full time daycare it was $2400 a month(one newborn and one toddler = alot of money..going to work was not worth it for me). Since you are giving her all the daily amenities, I think it would be safe to say maybe a hundred bucks a week? It would be all cash in her pocket for the most part.
You are providing meals as well, I am assuming, so she wont have the as an expense, unless she dines out alone every once and a while. That will give her money to go see a movie or an art exhibit, something fun on the weekends.
If she is making money with her art. Let that be hers. Shouldnt be put into your equation. I dont put my moms pay into mine. she doesnt live with us but it is the same theory. Thats her ''hobby'' income on her own.
Oh just saw your line about live in non family nannies...They make ALOT. I had a GF that did that through college with a family. She got paid more then I was making working high up at Nordstroms. I had thought about leaving and taking on a full timer nanny job...I wasnt a kid person back then though. I was making almost $2700 back in those days. She, I want to say got $3000, plus living area, off from the rest of the house.
Again, good for you! it is easy to except free help. But to recognize that she should receive money for her time, is a nice thing!!