Inspired by the HW thread, I have been thinking about this for a while.
Arthur works for the MoM so wont be on a pittance as a head of department, even a minor one as there would surely be grades?
They eat massive amounts of expensive food having three courses at every dinner, cooked breakfast every day. When they win a load of money, instead of saving it in the knowledge that they have 4 kids still at school they blow the lot on a holiday of a month in Egypt! At the beginning of CoS Mrs W takes some silver sickles and ONE gold galleon from their vault, and then happily blows 1000 galleons on a holiday rather than see them financially secure for the next few years.
Seems to me that the reason their kids are in second hand robes and reading second hand books is because they are shit with money, not that they dont have enough.
Books.....They have to have a new Miranda Goshawk book each year, so when F&G have finished with them they could have passed one to Ron who passed it to Ginny and the older ones shared the remainder if they needed it, which is unlikely. Why have several unused copies of the same book in the family and still buy another one for the youngest student? Same with robes. Rons dress robes are second hand, but Mrs W says that Arthur has some for parties so why not lend him those (with some magically performed alternations if needed) rather than pod out for horrible second hand ones? Actually, thinking further, Mrs W is supposed to be a whizz at household stuff so why doesnt she transfigure the second hand stuff to look better?
They just throw good money after bad!