This is ASSUMING THAT YOU HAVE AN iPHONE 3G or 3G S w/ UNLTD INTEGRATED MESSAGING:

+$XXX.XX Voice Rate Plan by-the-minutes (Monthly)
+$50.00 3G Data + Unlimited Messaging for iPhone (Monthly)
+$9.95 Add-a-line to the Family Plan (Monthly)
+$30.00 Another 3G Data for iPhone (Monthly)
+$XX.XX (Optional) Messaging Plan for iPhone #2 (Monthly)
(+$36.00) (One-Time) Activation Fee
____________________________________________________
$89.95 + Voice Plan Price + Messaging Plan for iPhone 2
$125.95 for the first month with the new iPhone and add above opts.

Here’s a scenario where you have a $65/mo bill for a 700-mins family plan:

+$65.00 700 minutes Family Shared Anytime Minutes
+$30.00 Mom’s iPhone #1 - 3G Data for iPhone. (Required)
+$09.95 Line # 2 for Family Plan
+$30.00 Your iPhone #2 - 3G Data for iPhone. (Required)
+$34.00 Entire family plan gets unlimited texts (per month)
___________________________________________________
$168.95 Your total per month.

Add $36.00 one time every month a new iPhone 3G/3G S device is activated. Add government
and state regulatory fees. Prices may change and vary
upon location. This information should only be used as a
reference. Consult your nearest AT&T Consumer Store
location for more information on billing for your AT&T service.

ADDED BONUS: Have many people that text oh-so-often? Get the $34/mo FAMILY UNLIMITED MESSAGING plan. It’s more worth it than getting 2-5 separate messaging plans, right? Send text, video, pictures, you name it to ANYONE in the family or ANYONE on ANY NETWORK! (Domestic U.S. applies. Others are subject to int’l charge.)

If you are using the original iPhone, please make the following corrections:

Data Plan: -$10
Text Plan: +$5

I hope the information above helped you. As always, visit an AT&T location nearby if you have any questions, as they may help you better than I may have.
References :
I have been an AT&T customer for 3 years or so and an iPhone customer for over 2 years. A wireless customer HAS to know his/her billing information, of course.