i am a student,my 13 year old daughter lives with and my 16 year old daughter stays wih me at the weekend, my 21 year old son who is also a student and his girlfriend who works (shop job) . We are arguing with him & his girlfriend how much should they contribute (money wise) to living with me, they also use our supplys aswell ! how much is a fair amount from them as a couple to give me each week. please help.