That calculator doesn't apply for people with older unlimited plans. The truth is if you have grandfathered unlimited plans and you want to keep it DO NOT UPGRADE WITH NEXT.
If you keep unlimited and upgrade with 2 year contract, you get a $450 subsidy on the phone which you essentially pay over 2 years. The subsidy is built into the cost of your plan. For example, my plan is $70 a month for unlimited data, 450 voice, 1500 SMS. 16gb iPhone is $200. Total cost over 24 months is $1880.
But if you keep unlimited and upgrade with NEXT you end up paying full retail price of the phone over 2 years in addition to the $450 subsidy that's built into the plan. AT&T obfuscates the subsidy in your older service plans. For example, my plan stays at $70 a month over two years but now I pay full price $650 for the phone. Total cost over 24 months is $2330