] [Fanny] "But then if Mrs. Dashwood should live fifteen years we shall be completely taken in."

] [Fanny] "...people always live for ever when there is any annuity to be paid them..."

I had also marked these as LOL lines for me, even as I cringed at their total want of every propoer feeling!

] Can someone explain what John means when he says "her life cannot be worth half that purchase," as he is talking about a 40-ish woman? Is he just going with the flow of Fanny's conversation (I am assuming he can't really mean what I think he means)?

I think that's exactly what he means--that she could not be expected to live until age 55! or even to age 50 if he literally means ' half that purchase'

It seems that 55 was thought to be a 'ripe old age' by the younger set, because later in Ch. 8 after they meet Colonel Brandon, Marianne says of the 35 year old colonel: " I know very well that Colonel Brandon is not old enough to make his friends yet apprehensive of losing him in the course of nature. He may live twenty years longer."