I wish you joy. If you love Mr. Darcy half as well as I do my dear Wickham, you must be very happy. It is a great comfort to have you so rich; and when you have nothing else to do, I hope you will think of us. I am sure Wickham would like a place at court very much, and I do not think we shall have quite money enough to live upon without some help. Any place would do of about three or hour hundred a year; but, however, do not speak to Mr. Darcy about it, if you had rather not.
Your, etc." (Chapter 61)
This letter sent from Lydia to Elizabeth gives a pretty insightful look into Lydia's character. While she does congratulate Elizabeth on her engagement, the sole purpose of this letter comes back to money. Lydia really is the product of Mr. and Mrs. Bennet's ill parenting. Much like her parents, Lydia's sole focus is on money, material possessions, and social standing. Lydia marries Wickham for the same reasons as Mr. and Mrs. Bennet: looks and good humour. While we see that Lizzy is not looking for a marriage of convenience (if she were, Collins would have been the perfect suitor), it is she who ends up with the wealth. It would be interesting to see how this step up in social standing changes how Elizabeth's family views her, as well as how they view the role they are supposed to play in relation to Elizabeth and Darcy's new union.