Look at all the people who were inconvenienced, to say the least, by Lydia's rash behavior. Her father had to go to London to try to find her. Lizzy and the Gardiners had to put an early end to a long-awaited vacation, and the Gardiners had to stay at Longbourn for a while, instead of taking their children home, until things settled down a bit. Col. Forster's reputation probably suffered, to say nothing of the reputation of Lydia's sisters, who in that society could very well have ended up all unmarried because of Lydia's perceived immorality. The Gardiners had the expense of drawing up the legalities and paying for the wedding, however small it may have been. And on top of it all, Lydia leaves some of her belongings behind with the Forsters -- who are in temporary military quarters -- and asks that the maid repair one of her gowns before packing it up!