Here’s an approach that worked for someone very close to me,
Daughter: “Dad, Mum, me and Neil have decided to go round the world, we’re going to Greece or Cyprus, then Egypt, then India, Thailand, Australia, New Zealand, we’ll stay there and work for a few months because neil has contacts, then Cook Islands, Mexico, L.A., New York and back to England. I have to have £2000 in the bank to get a work visa in New Zealand so with my tickets it will be about £4,400, call it £5K and if you want to throw in some extra for spending money it’ll be welcome.
Mum (horified): I will not see you for over a year.
Me (Knowing once Gabby’s mind is made up there’s no stopping her so I may as well kiss the money goodbye): Why the chuff do you want to go to The Cook Islands.
Daughter: Nobody I know has ever been there.
You just can’t argue with such reasoning.