Does anyone know how much the following will be?

My girlfriend currently has AT&T and her contract just ended. I am going to get the iPhone 3G. We want to be on the same plan. She needs a new phone, but probably will not get the iPhone 3G. How much will it be a month (the cheapest plan) for a Family Plan where one person has the iPhone (me) and the other doesn't (my girlfriend)? Also, can we set up this plan at a Apple store or only at a AT&T store?

Thank you.