Quote an example of Austen's use of sarcastice humor with respect to Collins? What character trait is Austen mocking?

I'm not sure what page this is (mine won't match yours) but in Chapter 22 Elizabeth thanks Miss.Lucas for talking to mr. Collins,

"It keeps him in good humour,'' said she, "and I am more obliged to you than I can express.''

Austen is mocking the fact that Collins is a pompous blowhard who loves to hear himself talk.