How much does it does data cost for an iPhone 4?

I'm looking to see how much the data plan for the iphone 4 is when you add it one to an existing line. I replacing it for an old samsung phone. How much will the add-on cost? I don't need an exact answer, just something close enough. If it matters the network is AT&T

With a discount in your contract, the phone will be free, without one, it might be $100 or $200. If you got an iPhone 4s, with a discount it will be $100, without one I think $200. I got an iPhone 5 with a discount recently and it costed me $200, without one it would have cost me $650. But the price for the phones varies at certain stores and locations.