How are you doing the calculations? With a 6 GB AT&T Mobile Share plan, the BYOD rate comes out to $360 less per line, which is less than the total subsidy over two years, assuming the $450 iPhone subsidy per line. However, with a data allowance of 10 GB or more, the price gap increases to $600 per line over two years. So, the contract price can work out less, if you upgrade devices every two years.
However, this all goes out the window though if, for example, I bring along my current phone, and only add one new phone or zero new phones.
I currently use a $30 prepaid plan with a 5 GB soft data cap. No individual on-contract plan that I've looked at even comes close, even after deducting the subsidy value. Because I own the phone outright, I have the flexibility to switch plans at any time. Whether family or individual, prepaid or postpaid, contract-free or on-contract, all options remain open.