The rent I was paying in Norwich when I was a student was £290 a month, which was a double room in a four-person house. There was, of course, food and bills on top of that.
Now that I've moved home, I'm going to be paying rent to my mum. When i was in my gap year (before I went to uni), I was earning a fair amount, and paid £150 a month as rent. it's not as much as you would expect to pay, but it went towards covering her expenses because of me (food, washing, heating, that kind of thing).
I think I'll probably be paying about the same again now. I'm away during the week, though, so she doesn't have to feed me then.
Like Bluegem has said, even if it's just £100, it's £100 more than they had before, and it's good to show willing to support yourself