What company is the cheapest for iPhone?

I want to get an iPhone 4s but I don't know what company would be the cheapest? My parents don't really like paying a lot of money on a phone bill. Right now I have a phone on the Metro PCS company and everything is unlimited. I want a plan were text messaging and the web is unlimited because that's what I spend most of the time using but that could also be cheap. And i don't really know what people mean when the company goes by data please explain. Thanks!

Added (1). I want to know about the plans as well! Can someone help me with them too?

Best bet go to ebay. I got my iPhone 4s brand new on there 32Gb for £450:) when it first came out

Verizon
I have verizon, I'm very happy with it.
BTW if you want to buy an Iphone cheap there are stores called "the apple orchard" they resell apple products (new or used, mostly used) i got my mac there, and my parents got their iphones there.

Verizon. For my thirteenth birthday in September I got a new iPhone 4S. It was $200 but its probably cheaper now because the iPhone 5 came out. My little sister got an iPhone 4 for $100 at the Verizon store!