I would say that a crack would affect its value but nowhere near as much as a front screen crack. The defination of good working order is always a difficult one particularly when it comes to phones.
Optus give a pretty clear list of possible issues and a a case crack is one of them.
IMO a case crack is potentially worse than a screen crack as phones are designed to swap screens but not designed to swap cases. I suggest you take it in for evaluation. Optus will either accept it or tell you exactly how much you need to pay to qualify. You could also see what other vendors would charge to fix it.