How much do you pay monthly for an iPhone 4s on AT&T or Verizon?

So I really want an iPhone 4s but if i get one I'm going to ask for my birthday and I don't want my parents to be paying a lot monthly. My friend got a 4s and she has Verizon and she said her grandparents pay only like 30$ monthly and she said she has unlimited texting and data. But I thought people payed like around 80$ monthly. What's cheaper AT&T or Verizon?

Verizon recently changed their plans.
$40 for a smart phone and $50 for 1GB of data. After taxes and fees you can expect to pay $120 to $130 per month for 1 smart phone with Verizon.

I would look into a prepaid smartphone that will not be an Iphone. You can get prepaid smart phones for as little as $80. The monthly plans are usually around $50 per month.

149 for sprint but thats with 2 lines. And sprint is unlimited data plans.