The good news: as long as Mr. Bennet can promise to send them a little money every year, they will be.
Mr. Bennet and Elizabeth discuss the fact that somebody else, probably Mr. Gardiner, must have paid a great deal of money to convince Mr. Wickham to marry Lydia only asking for such a small annual sum.
He thinks that it probably cost at least 10,000 pounds to make Mr. Wickham marry Lydia.
Comparing sums of money isn't as straightforward as multiplying by whatever, but believe us: that is a ton of money. Darcy gets 10,000 pounds a year from his estates, so, take an enormous yearly income of today—high six or low seven figures—and that's probably how much money we're talking about.
(Want more speculation about money in Austen? Here's a cool article that translates some of those pounds into dollars—but 1989 dollars, so the amounts would be even higher today.)
Mrs. Bennet is delighted that her daughter will be married and figures that it's perfectly okay for Mr. Gardiner to pay for it. (To be fair, she doesn't realize exactly how money is at stake.)