For that will be the tone after marriage, it's a foretaste of it.

And mother too, why should she be so lavish?
What will she have by the time she gets to Petersburg?
Three silver roubles or two 'paper ones' as _she_ says....

that old woman...

hm.

What does she expect to live upon in Petersburg afterwards?
She has her reasons already for guessing that she _could not_ live with Dounia after the marriage, even for the first few months.