IPhone 3G Question...?

my family and i, consisting of 3 phones altogether, have a chance to upgrade on 10/20. i was considering the iPhone 3G, now that's it's only $199. (my parent's wanted free phones) our current plan is 550 minutes for the whole family, we don't talk that much, and i have a separate $15 texing plan just for me. if we upgrade and get another two year plan, how much would the iPhone cost? and is the $30 data plan optional? thanks.