I'm a lurker, but we just went through this so I can contribute. You can add the iphone data plan to your existing plan for $30. That is for unlimited data (internet, emails) if you want text messages as well they come in $5- 200, $15 -1500, etc.
We upgraded my DH's phone to an iphone. Kept our family plan and added the data and some texts. No need to change your original minutes plan, just add the $30 data plan to your bill now and you can use the iphone
That being said, if you both want iphones, it would be $60 more a month plus text package, if you want that. At that point, you may re-evaluate your plans and go with something else. Also, don't think that the prices in the brochures are the only packages, get a good rep and ask the TOTAL price, don't let them talk you into one of the plans on the brochure, just becuase you think it's the only ones available !
HTH