I was having this conversation with my 14yo last night! Not planning on charging him now, but he was talking about the future being scary, what he will do etc and I said well it wasn't like I'd be kicking him out at 18! And it progressed into a cost for board.
I recon I spend about £400pm on groceries, cleaning stuff etc for the 3 of us (2 teens and I) so I said say £100pm for food etc. Plus I'm on single adult council tax reduction, so another adult adds £50pm to that. Plus a share of utilities. So £200pm.
For that he'd have the use of his room, plus communal rooms in a nice house. Food & use of kitchen. Use of washing & drying facilities and powders etc.
I'd anticipate travel costs to whatever would be about £50pw here.
So on approx £1k pm he'd have £600 left to cover clothes, tech, socialising, saving etc. Which seems plenty to me for that age & income level.
However I also said that I'd expect him to be a reasonable member of the household, clearing up after himself and communal chores, cooking for the household sometimes etc.