Babysitting, am i being used?

Ok, I am helping a friend out by babysitting his 2 year old son, sometimes i have him for 4 days, and overnights, i'm trying to be nice by just charging, 20 dollars a day, is that a reasonable, price? I'm supply, food juice, sometime he supplies the diapers, and wipes, sometimes i buy them, i think i'm getting duped, any suggestions?