In my case I bought a pristine iphone 4 (used) and now a used (clean) 4S outright since I wanted to use Tmo prepaid and Straighttalk.
The ip4 cost me about 400, just sold it unlocked for 350.
The 4S cost me 450$ unlocked (64GB) and I can switch between Tmo and Straighttalk ($45 unlimited everything plan suffices for my needs) as needed. My bills when I use Tmo prepaid are 10$ a monthsince I have wifi access 99% of the time.
Huge savings over the past 2 years already.
For family plans with grandfathered unlimited data like myself with 4 devices it's not cost effective to be on pre paid.