The Weasleys are not especially poor. It's just that Arthur works at a comparatively low paying job at the Ministry, and has a huge family to feed. Molly is a housewife and does not add to the income.
As to how poor they are, well, they are helped by not having to feed their children throughout the year. I think, when its just Molly and Arthur at home, they get on fine, even manage to save up some money for the Christmas and summer holidays, when the kids come home.
Summer is an especially hard time because of the need to buy school books and other items for up to five children at a time. As is evidenced in Harry Potter and the Chamber of Secrets, Chapter 4: At Flourish and Blotts
Harry enjoyed the breakneck journey down to the
Weasleys’ vault, but felt dreadful, far worse
than he had in Knockturn Alley, when it was
opened. There was a very small pile of silver
Sickles inside, and just one gold Galleon. Mrs.Weasley felt right into the corners before sweeping the whole lot into
Despite all this, the Weasleys do get along well enough. There is never a case of not having enough food to put on the table. In fact, Mrs. Wealsey is known for her wonderful cooking, always making enough to fill the stomachs of everyone at the table, and then some.
There's also the jackpot that Arthur won in Harry Potter and the Prisoner of Azkaban. The Weasleys spent that money to holiday in Egypt. If their financial situation was very tight, there would be no question of blowing the money away on a foreign trip.