It's not simple sarcasm. There's plenty of truth in it, so there's an element of knowing self-deprecation - not just on Elizabeth's part, but also Austen's, because the sentiment that would have been recognised by many of her readers.
Austen herself was in the same position as Elizabeth, with a tiny inheritance that provided no financial security. So P&P is as much a fantasy about finding financial safety as it is about finding a man.
For all the landscaped country houses and bonnets, it was a very brutal period for many women - and for men too, in other ways.