Will try to be as objective as possible and not give away which party I am.
Grown up DS& his wife and one DC. They have a combined income of about £1600. One room, free access to bathroom (when free of course) and kitchen, they have access to things such as milk/bread/if there are crisps/fruit etc but also replace those when they run out. Grown up DD charged nothing to stay although she may start uni in September (not doing anything right now, weekend job). So the money would be to cover extra heating/hot water/ (there's no water bill, just for hot water)/use of room/ some food but anything specific they have to buy/ electricity. Both shower every other day (about 10mins each), DC has small bath every night and of course there's extra washing/toiletries etc. How much a month would you ask for? They pay about £650 in personal bills eg phones, netflix (which everyone has access to) and they pay for the broadband/tv channels/WiFi for the house.