How much does an iPhone 3G cost per month with unlimited data and text messaging on a family plan?

I’m currently on a family plan. I’m interested in the iPhone 3G now because Apple introduced the iPhone 3GS and the original iPhone 3G is only $99 now. How much does it cost per month with unlimited data and text messaging? I’m on a family plan. Oh! and I’m currently not eligable for an upgrade, but can I just sign up for another 2 years and get the iPhone 3G for the $99?