In the beginning of Chamber of Secrets, the Weasleys and Harry get their book lists which have all the Lockhart books and one of the twins remarks that 5 sets of Lockhart's books won't come cheap. Molly also mentions that they'll be able to get a lot of Ginnys books secondhand.
But why wouldn't Ginny just use Ron's books from the previous year?
And as for the Lockhart books, there's only one teacher, Lockhart, so really they'd need two sets max, as Fred and George would need a set each being in the same class, but couldn't the books get passed around between the other siblings?
I know it seems trivial, but for a poor family you think they would be a bit thriftier about the books.
On another note, Dumbledore mentions to Tom Riddle, in his memory in The Half Blood Prince, that Hogwarts has a fund for those who can't afford stuff. Did the Weasleys ever get anything from that fund?