How much does an Iphone 4 plan cost per month?

Right now i have a Samsung Rouge. I want to get an Iphone 4. I plan on buying an Iphone 4 off of ebay. I know that the phone plans are not the same so I want to know the difference per month. If i buy one off of ebay do i have have to redo my contract with verizon or can i just up my plan without redoing my contract?