I'm the adult who lived with parents until earlier this year.
I did £400 a month, which was essentially just covering costs. It covered 1/3 of all bills + £100 food because they did 95% of shopping. They viewed it as an 'at cost' arrangement, were happy doing that if I was making active steps for seriously saving for FTB. They were not in need of income and wouldn't otherwise have rented out my room.
My take home pay was £1600, I put £800-£1000 each month into savings, £400 to my parents, and the remaining £2-400 a month was for myself- of which about half went on the car, the rest other bits and bobs.
You could say they were 'making' on me to a v small extent as fixed charges like council tax, broadband, Netflix, tv licence, etc etc, were not dependent on the number of consumers. But I got to stay in a much nicer house than I could otherwise afford for less than half the cost of my one-bed rental from 4 years ago.
It worked out very happily for everyone. Especially now for my parents now i've finally left!