how much board to charge children

My child is having a gap year from uni and is earning about $1100 clear a fortnight in a job for this year.

consumes 2 meals a day . we do the washing/ironing and the usual clean up after. no real car costs but of course electricity , internet, gas etc.

Any thoughts

side note....we paid $18 k for full board at uni so we reckon a bit of board is appropriate.