How much child support should we be getting from our dad?

Right now he is giving us $100 a week for me and my brother we live with our mom now we didn't get along with our stepmom i tried my best to get along but she would pick fights over nothing and according to a shrink its her fault my little brother was so depressed and it was we were told we needed to move out. I give my brother $20 a week and the rest almost all goes to gas but he needs clothes so i am thinking thats not enough money i am not out to get him for all his money i just need a little more help since i cant find a job. Im 18 and he is 16 btw