As I'm currently getting back into PT, I had to look at my options for insurance recently. I looked outside of Fitness Australia/Guild (very briefly; there may be much better or much worse deals around than what I looked into) and was looking at about $800 for insurance without going through the fitness bodies. The savings there appear to be in not having to do courses to obtain and maintain your insurance, rather than the immediate cost of insurance. Since I already had current FA registration and Guild insurance through FA would cost me very little up front, I decided to stick with them for this term. Depending on what courses I do/don't do over the next year, I might go elsewhere next term.
All up, FA membership and Guild insurance is about $300-400. If you've already spent the money getting certified or getting CEC's, I'm not sure that going elsewhere for insurance right now would be a great idea. If you haven't spent your money doing the stuff that would qualify you for FA/Guild, then it's time to look elsewhere, but don't expect it to be individually cheaper than FA/Guild.