If you can live with the VM network with 300 min/month, then you can see that the savings are the clear winner here:
Virgin Mobile iPhone 5 16GB - $650
plan - $30/mo (300 min)
after a year - $1010 ($2.77/day)
after 2 years - $1370 ($1.88/day)
after 3 years - $1730 ($1.58/day)
after 4 years - $2090 ($1.43/day)
Virgin Mobile iPhone 5 64GB - $850
plan - $30/mo (300 min)
after a year - $1210 ($3.32/day)
after 2 years - $1570 ($2.15/day)
after 3 years - $1930 ($1.76/day)
after 4 years - $2290 ($1.57/day)
Straight Talk iPhone 5 16GB - $650
plan - $495/year (unlimited) + $15 nano SIM card
after a year - $1160 ($3.18/day)
after 2 years - $1655 ($2.27/day)
after 3 years - $2150 ($1.96/day)
after 4 years - $2645 ($1.81/day)
Sprint iPhone 5 16GB - $200
plan - $80/mo (450 min)
after a year - $1160 ($3.18 per day)
after 2 years - $2120 ($2.90 per day)
after 3 years - $3080 ($2.81 per day)
after 4 years - $4040 ($2.77 per day)
Of course, the iPhone 5 prices are based on current price points, so I'm guessing that they'd stay the same once the unlocked version comes out. I'm also guessing that ST will come out with a nano SIM at the same price point.
The way I figured it, add up the price of the phone with the plan, and then calculate the overall cost over a period of time. So, even though the device is cheaper with a contract carrier, it'll be way more expensive overall.
But for some reason, "unlocked" and "no contract" are sometimes confused - these are not the same thing.