How much rent should i pay my parents?

[Dagger]

Im 19, have a part time job which earns me �150 a month. I Spend roughly 3 - 4 nights a week at my parents house. The only meals i eat at there house is my evening meal, which i only have 3 times a week maximum. I dont use the telephone, nor do i watch the TV, or use the internet/computer. They are demanding �100 a month from me, which considering the amount of time i spend living there, and electric/food i consume, i do not deem it fair to have 2/3 of my pay paid in rent to them. How much do u recommend i pay them?