in pride and predjudice why is the exchange between darcy and mrs. bigley amusing to elizabeth

Well the exchange is amusing to her as to her it is obvious that miss Bingley is flirting and has her designs on Mr Darcy and that the said Mr Darcy is as uncivil as can be possible. Loving all things ridiculous and "dearly loving a laugh" this situation whereby a proud woman would stoop to put up with the arrogance and even snubbery of a man simply because she wishes to marry him is cause enough for amusement for her.

She has no particular liking fro either and theefore can amuse herself at their expense without feeling teh pain she would have felt had either been dear to her.

Elizabeth is amused because it gives her an opportunity to banter ans indulge in sarcasm. She's very good at putting people in their place without drawing attention to what she's doing. The more Mrs. Bingley strokes Darcy's ego, the more Elizabeth indulges in quietly making fun of them all.