How much will the iPhone 4g cost per month on a family plan?

I want to get the iPhone on my plan. Right now I have 6 people on the plan and I pay $170 a month. I was wondering how much it would cost to get it. I already have the unlimited text message plan per month.|||I think it is limited to 5 people per plan. Usually about $100 for unlimited text, calling and data and then about 10 for each extra person. But I think that you can't have unlimited when you split a family plan. Hope this helps