I want to upgrade to an IPhone?

I want to upgrade to an iPhone but first I want to know how much extra it will cost per month. I have a regular 2 year contract. My phone doesn't have a data package and I'm not sure but I think its $30 per month as it is. I was wondering if that sounds right and how much EXTRA it will cost to upgrade to an iPhone. Also will it depend on which iPhone I get or how many GB's the phone is?

Thanks!

Added (1). My carrier is Verizon if that makes a difference.

Depends on the carrier, remember that the more storage an Iphone haves ( 16, 32, 64 GB), it will increase it's cost in more or less 100 dollars, the contract should be either around 30 to 50 Dollars, but then again, this depends on the carrier and data package you choose I believe.