Okay I have to admit this one has always confused me to no end. Arthur works in the Ministry. That is basically the wizard parliment, goverment, and law enforcement. They make good money. Yet in the books and films The Weasleys are dirt poor.
I get havin seven kids and a stay at home mother can put a cramp in your budget. However he works in the government! He should be makin enougg to be at least upper middle class or just middle class. Unless he's the equivalent of the mail room man (post office people aren't paid that shaby either) there is a severely messed up rate exchange with wizard money. Or its simple and Molly and/or Arthur have a drinking or gambling problem that isn't discussed in book or film. With seven kids I wouldn't blame them if they had magical visectomies and tube tying done.