Lydia cost ninety pounds a year before she was married:
`He would scarcely be ten pounds a year the loser, by the hundred that was to be paid them; for, what with her board and pocket allowance, and the continual presents in money which passed to her through her mother's hands, Lydia's expences had been very little within that sum.'

While Elizabeth would no doubt have been more economical than Lydia, it's unlikely that even she could appear in the manner befitting Mrs Darcy on such a sum. Mr Darcy would bestow a good allowance on his wife from pride as well as from being love-struck.

And out of this she would supplement the Wickhams:
`Such relief, however, as it was in her power to afford, by the practice of what might be called economy in her own private expences, she frequently sent them.'

I get a definite feeling of irony going on here, in the phrase `what might be called economy in her own private expences'. But what JA is being ironical about, I don't know. Perhaps Elizabeth was doing it very discreetly, so her husband didn't find out?