How much is an iPhone bill?

I wanted to buy an iPhone 3GS because its only $50 now. I was wondering how much i would pay. My family is already on a plan and i was wondering if i could still keep my plan which has unlimited texting and however many minutes and just add the 2GB data plan for $25. Or would my plan be separate from my familys? If anyone has any personal experience it would be much appreciated :)