"Then" is used in an "if-then" scenario, to dictate chronological order. In other words, "If you jump off, then you will fall," or "he was attacked and then retreated."
"Than" is used for comparisons. In other words, "The egg was bigger than the pea," or "I'm stronger than you."
When Lady Bracknell says: "A hundred and thirty thousand pounds! And in the funds! Miss Cardew seems to me a most attractive young lady, now that I look at her"
Even though the whole scene is about trying to show that Miss Cardew is eligible for marriage, Lady Bracknell doesn't seem to accept it until she learns how much money she has. Suddenly then she seems to be a viable candidate for marriage.
