When I first started work back in the late 80's, I was getting about £27 a week and £10 of that went to my mum.
I was left with £17 which doesn't sound much but it kept me in smokes and I had one or two nights out a week.
If I was in my mums position now and had my own kid, I'd charge them just for food, but the more they eat the more they pay.