I think it is several factors...

Arthur loves his job. He picked it based on his passion and fascination for muggle things, not for how much money it would make him.

He works for the ministry of magic, so he is basically a public/civil servant. Less opportunity for cash than in a private/corporate job.

His particularly department is muggle-centered. As *most* of the wizarding world is not very concerned with muggles, it is probably an underfunded program. 

And he has one million children. Children are expensive. 

I mean, they have a huge house and get by, but they just can't afford a ton of 'extras' for their kids. They are not quite living pay check to pay check, but they probably don't have much in the way of 'back up' cash if Arthur ever lost his job or something.  Ron gets all the hand-me-downs which is normal in families with tons of kids. Oldest son got all the new things, was the first through school, and Ron is at the tail end of a long line of brothers, his experience is typical of youngest brothers. Ron's particular experience is *especially* pronounced when contrasted to Harry's situation. Who shows up in the wizard world with a huge inheritance and buys everything new and doesn't need to worry about cash or sharing things with siblings or anything like that. 

I think the depiction of the family situation is pretty realistic ^_^