Frank, I am in the same situation as you are. My husband and I both have iPhones under a family plan (1400 shared minutes). This might help, I just studied my itemized AT&T Wireless bill...
For the main number, the rate plan charge (for the entire family plan but charged to this one number only) is $80 (which is basically the main charge for the plan we have, if you have fewer minutes, this would be less in your case). That includes the basic features (call forwarding, call waiting, caller ID, etc). Under "Other Services," I have a host of feautures such as Anytime Minutes, Rollover, etc but they all cost 0.00. There is one feature that costs $20, entitled "iPhone Data + 200" (the 200 being the 200 included text messages that come with the iPhone data plan). My line itself costs a total of $100, not including taxes, etc.
When I scroll down to the charges for my husband's number, he has a 9.99 charge under "Monthly Service Charges" and the item specifies it's a secondary line. He also has the $20 charge under "Other Services" for "iPhone Data +200." So his line, before taxes, etc costs $29.99.
I hope this helps. Let me know if you have any other questions about the bill and I"ll be happy to compare it against my bill.