How much would i pay for the iphone 3g s?

i had the blackjack 2, then bought the LG Vu and i hate it, so i want to buy the iphone 3g s. its burly going to be one year with my 2 year contract... i pay from 60-75 dollars a month... how much would i have to pay for the phone and how much a month. do i have to get the data plan by force.