I would "suggest" that any son/daughter pays about say 25% of their net wage even if you didn't really need it as it gets them used to paying bills/ budgetting for the time when they do fly the nest,and if you stuff the money in a separate account and they come cap in hand for help with a mortgage deposit you could then give them a lump sum back......... or treat yourself
p.s I was paying £2.50 a week many years ago but I was only getting £9 a week .