AT&T: $325 - $10 for each month you've had service.
Verizon: $350 - $10 for each mont you've had service.
Since the subsidy is higher than the ETF for the iPhone, that's why carriers secretly wish people would be other phones instead because they would make $100 more off of you. $100 per phone * millions of users is not chump change, especially when you start everyone using smartphones in a couple of years.
This is why Verizon continues to push Android and AT&T is taking a chance with Windows Phone; iPhones cost them money relative to other platforms (and I haven't even gotten into the fact that Android and Windows Phone lets them preload their own branded apps onto the device, leading to more revenue for them)
Voice already isn't the primary use of smartphones, SMS revenues are beginning to dry up due to iMessage, BBM, Whatsapp and other messaging services, and carriers haven't figured out how to charge for data yet other than silly caps. And when a Voice over LTE arrives, there will be zero difference between that and a Skype call, it's all IP Telephony that's at that point (so why should I pay $40 for 450 minutes when a Skype package is FAR cheaper?)
Regardless, a lot goes on in the mind of a wireless carrier. If we aren't aware of what they are doing, we are going to get screwed.