£633 here. I think, though, it depends on the rest of their outgoings, their age and their earnings. To rent a 1 bedroom flat near the station (which is 10 miles away from our home) it would cost £650pcm. So £633 including all the bills and food is pretty good really.
BUT, it costs over £5000 for a season ticket for the train here so that'd be £400 ish per month on top of the £633 or the £650 if they were private renting. If they're just starting out a monthly take home of £1033 per month means earning something in the region of £23-£25K per year to allow for saving and going out.
Mind you my cousin did charge the going rate for her girls when they started work - in this case she'd have kept £100 a week and saved the rest (because she knew they were pretty dire at saving) so when they got to 21 they had a nice little sum saved in secret.
I don't think it does any harm to let the kids see what they 'should' be paying - then see what they can get privately for the same amount. But I won't be charging DS that if he were working now. Our mortgage will be gone when he starts work. Might agree with him an amount to help towards OUR bills plus a bit for me to save - so he's actually handing over savings money regularly instead.