Depends on when and how you use it:
I have done Optus, Virgin, 3 and now Telstra. I use my mobile almost exclusively and for almost everything. Including hammering rivets. The rounded back of the iPhone is especially ideal.
I somehow always paid around $120-$300 per month. No matter the plan or phone or network. So I got the iPhone ultimate plan on Telstra and I must say technically speaking it is excellent. I always pay $150 a month. Fixed. At least I can budget it and have no surprises. Internet is always available, fast and on. Yes, I have jailbroken the iPhone so I can tether it to my laptop, though I have only done it two or three times in as many months.