]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?

Paying the annuity is, essentially, like buying more years for Mrs. Dashwood; as Fanny points out, the prospect of money is a great extender of life. According to her estimate, the Dashwoods will be purchasing fifteen more years of life for their stepmother, whereupon John objects that she can't possibly live half that long--i.e. the purchase will be a much smaller one.

]Isn't this the most appalling discussion (so why am I laughing?!).

They're so absurdly crass it's hard not to laugh! Though I must admit I feel ashamed of myself afterwards.