It's actually quite hard to use more than about 300MB on a smart phone, unless you do audio or video streaming, most people I speak to seem to be below 200MB and to their surprise found they over-spent on a larger data plan than what they need. And wouldn't data caps be preferable to carriers selectively banning data intensive app?
I think that even if you take the data cap into account, AT&T still massively overcharges for the iPhone.
Subsidisation here works differently. On Optus for iPhone 32GB for example, for voice plans, $19($16USD), about 50 minutes and 100MB, for $49($40USD) about 400 minutes and 250M. They don't charge for the handset upfront but it comes out of your bill monthly $34($28USD) for the $19 plan, $19($16USD) for the $49 plan, that doesn't quite cover the cost of the phone though it's still subsidised by the voice plan. They also have an all you can eat plan for $99 ($82USD), which contains unlimited call minutes, $0 for the phone entirely subsidised and 1.5GB of data, hardly any iPhone user is going to exceed that much data, and this is only $12 more than AT&T's least expensive plan, hehe.
If the voice plan doesn't contain enough free data for you, they offer additional data plan, $20($16USD) for 1GB, $45($37USD) for 6GB, and these plans will also enable tethering on the phone. 6GB on a smart phone is practically unlimited! It's not realistic that you could pass that unless you use tethering.
Of course this is Optus, their network drives people crazy with it's dropped calls and how often it drops to GPRS because their 3G sucks. But with these prices, they have the largest market share with iPhone users.
Telstra has an awesome network with 21mbit 3G, 99% of the population covered, that is a ground area of 3 times the size of texas. But they sure make you pay for it. Lets see, Telstra, $49($40USD) for 250 minutes and $550($454USD) for the phone upfront cost. Hardly any data is included in the voice plan. Their data plans, 300MB $30($24USD), $60($49USD) 1GB, $90($74USD) 5GB. Yikes! Sure with them you can get 3G in the middle of no where with nothing but kangaroos to share the experience with, but at this price, well most people are picking Optus. They're offering these absurd prices, not because they have a monopoly on the product like AT&T, but because they have a far superior network that's 5 bars 3G pretty much all the time and hardly ever drops a call. (which, on a different topic, it's performance on telstra is good evidence that nothing is wrong with the iPhone's radio).
No doubt it wouldn't be so cheap here, if not for the fact that Apple sold the phone unlocked in Apple stores, and that the 4 main carriers offer the iPhone on contract, so there is real competition. The main reason this situation occurred, is because Telstra, being the arrogant and pig headed company they are, was the only carrier to have EDGE back in 2007, rejected the iPhone as they were upset that Apple wouldn't allow them to put their software and services on the phone. And famously publicly told Apple to stick to their knitting! (an expression, basically saying, they should stick to making iPods and not mess around in markets they have no idea about).