As we know the Weasley family are quite poor. Not poor enough that they don't have anything to eat but still so poor that they have to buysbuy second-hand clothes and books for their children.
Yet, when they win 700 galleons from the lottery they burn most of them on a trip to Egypt. While it must have been fun it seemed extremely unwise. Afterwards they are poor again - Ron has to wear a second hand and extremely old-fashioned robe on the Yule Ball.
So why did they spend so much money on a single trip?