I think Mrs Dashwood might have found herself better off in Devon than she previously imagined on £500 per year.

Re: C18th Devon,

"On the whole, it was reckoned that the cost of living in Devon was only about two-thirds of that in London. Four hundred pounds a year in Devon would go as far as six hundred pounds in London." (Hoskins, Devon and its People, 1959)

I also have somewhere (!) a reference to Devon's seaside towns being much cheaper to live in c1800 than places like Ramsgate or Margate.

I have no idea of the relative cost between Sussex and London but if someone has this information (Julie?) then it might be possible to judge whether they will be better off on £500 in Devon than they would be on £500 in Sussex.